Aircraft with thermal energy storage system for multiple heat loads

ABSTRACT

A thermal energy system for use with an aircraft includes a cooling loop and a cooler. The cooling loop includes a fluid conduit and a pump configured to move fluid through the fluid conduit to transfer heat from a heat source to the fluid in the fluid conduit to cool the heat source. The cooler includes an air-stream heat exchanger located in a duct and is in thermal communication with the fluid conduit to transfer heat between the fluid in the cooling loop and the air passing through the duct.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalPatent Application No. 63/086,804, filed 2 Oct. 2020, the disclosure ofwhich is now expressly incorporated herein by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to aircraft, and morespecifically to aircraft with thermal energy systems for cooling heatgenerating components of the aircraft.

BACKGROUND

Gas turbine engines are used to power aircraft, watercraft, powergenerators, and the like. Gas turbine engines typically include acompressor, a combustor, and a turbine. The compressor compresses airdrawn into the engine and delivers high pressure air to the combustor.In the combustor, fuel is mixed with the high pressure air and isignited. Products of the combustion reaction in the combustor aredirected into the turbine where work is extracted to drive thecompressor and, sometimes, an output shaft. Left-over products of thecombustion are exhausted out of the turbine and may provide thrust insome applications.

Some aircraft may include a hybrid electric propulsion system that mayuse one or more electric machines, such as generators and motors, toprovide power to the aircraft. To reject waste heat produced by theengine, aircraft oil/hydraulic heat loads, power electronics, and/or theelectric machines, the aircraft may use a ram-air heat exchanger toreject waste heat to the surrounding air in the atmosphere. However,when the aircraft is grounded or taxiing, the air may be relatively warmrelative to the components to be cooled. Moreover, there may be littleto no ram air pressure to force the air through the ram-air heatexchanger when the aircraft is grounded or taxiing, limiting the wasteheat rejection.

As a result, the ram air duct and heat exchanger may be relatively largeto achieve the large mass flow rates to effectively transfer heat to theair in the atmosphere at low ram pressure conditions. Conversely, atcruise altitude, the air temperature is cooler and the ram air pressuredifference across the ram-air heat exchanger may have increased due tohigher flight speed, which would allow for a smaller ram-air heatexchanger to be used, but for the insufficient cooling rate whilegrounded and taxiing.

To enable a smaller ram-air heat exchanger when the aircraft is groundedor taxiing, the aircraft may include a blower to draw air through theram-air heat exchanger, adding weight to the aircraft. As a result,providing a thermal energy system that enables a ram-air heat exchangerconfigured to maximize fuel efficiency and decrease the added weight tothe aircraft presents design challenges.

SUMMARY

The present disclosure may comprise one or more of the followingfeatures and combinations thereof.

A thermal energy system for use with an aircraft may include a firstheat source, a second heat source, a cooling loop, a cooler, and athermal-storage fuel system. The second heat source may be configured tooperate at a lower temperature than the first heat source during use ofthe thermal energy system.

In some embodiments, the cooling loop may have a fluid conduit, a pump,a compressor, and a first expansion valve. The pump may be configured tomove fluid through the fluid conduit to transfer heat from the firstheat source and the second heat source to the fluid to cool the firstheat source and the second heat source. The compressor may be locateddownstream of the second heat source. The first expansion valve may belocated upstream of the second heat source.

In some embodiments, the cooler may include a duct and an air-streamheat exchanger. The duct may be configured to conduct air through theduct. The air-stream heat exchanger may be located in the duct and inthermal communication with the fluid in the cooling loop to transferheat between the cooling loop and the air conducted through the duct.

In some embodiments, the thermal-storage fuel system may include a firstfuel tank and a fuel-tank heat exchanger. The first fuel tank may beconfigured to store fuel therein. The fuel-tank heat exchanger may be inthermal communication with the fluid and configured to transfer heatbetween the fluid in the cooling loop and fuel stored in the first fueltank.

In some embodiments, the thermal-storage fuel system may further includea second fuel tank. The second fuel tank may be in fluid communicationwith the first fuel tank.

In some embodiments, the second fuel tank may be configured to storefuel at a temperature that is different from a temperature of the fuelin the first fuel tank. In some embodiments, the second fuel tank may beconfigured to store fuel at a temperature lower than a temperature ofthe fuel in the first fuel tank.

In some embodiments, the thermal-storage fuel system may further includean engine-fuel unit. The engine-fuel unit may be configured to receivefuel from the second fuel tank and deliver the fuel to an engine. Theengine-fuel unit may be in thermal communication with the cooling loopto transfer heat between the fluid in the cooling loop and the fuel inthe engine-fuel unit.

In some embodiments, the engine-fuel unit may include a valve system andan engine-fuel heat exchanger. The valve system may be in fluidcommunication with the second fuel tank. The engine-fuel heat exchangermay be in thermal communication with the valve system and the coolingloop to transfer heat between the fluid in the cooling loop and the fuelfrom the second fuel tank. In some embodiments, the valve system may beconfigured vary a flow of fuel through the engine-fuel heat exchanger todeliver fuel to the engine at an engine-fuel unit predeterminedthreshold temperature.

In some embodiments, the valve system may include a mix valve, a firstconduit, and a second conduit. The first conduit may be in fluidcommunication between the mix valve and the second fuel tank to deliverfuel having a first temperature to the mix valve. The second conduit maybe in fluid communication between the mix valve and the second fueltank.

In some embodiments, the engine-fuel heat exchanger may be in thermalcommunication with the second conduit to cause the second conduit todeliver fuel having a second temperature to the mix valve. In someembodiments, the mix valve may be configured to vary a first flow rateof fuel from the first conduit and a second flow rate of fuel from thesecond conduit to provide a mixed stream of fuel.

In some embodiments, mix stream of fuel may have a third temperature.The third temperature may be less than or equal to the engine-fuel unitpredetermined threshold temperature.

In some embodiments, the cooling loop may be structured to conduct thefluid heated from the first heat source and the second heat source firstthrough the engine-fuel heat exchanger, through the air-stream heatexchanger after the engine-fuel heat exchanger, and through thefuel-tank heat exchanger after the air-stream heat exchanger.

In some embodiments, the control system may be configured to measure theheat transfer between the fluid in the cooling loop and the fuel in thefirst fuel tank. The control system may be configured to compare theheat transfer measured to a predetermined heat rejection schedule. Thecontrol system may be configured to vary the flow of fluid in thecooling loop through the fuel-tank heat exchanger in response to theheat transfer measured being different from the predetermined heatrejection schedule

In some embodiments, the cooling loop may be structured to conduct fluidthrough the first expansion valve, through the second heat source afterthe first expansion valve, and through the compressor after the secondheat source. The cooling loop may be structured as such to lower atemperature of the fluid in the cooling loop so that heat produced bythe second heat source is removed from the cooling loop. The coolingloop may be structured as such to lower a temperature of the fluid inthe cooling loop before conducting the fluid through the fuel-tank heatexchanger.

In some embodiments, the cooling loop may further include a secondexpansion valve. The second expansion valve may be located upstream ofthe fuel-tank heat exchanger.

In some embodiments, the cooling loop may be structured to conduct fluidthrough the second expansion valve, through the fuel-tank heat exchangerafter the second expansion valve, and through the compressor after thefuel-tank heat exchanger. In some embodiments, the cooling loop may bestructured to conduct fluid through the engine-fuel heat exchanger afterthe compressor and through the air-stream heat exchanger after theengine-fuel heat exchanger.

In some embodiments, the cooling loop may include a plurality of valves.The plurality of valves may be connected to the fluid conduit andconfigured to selectively cause at least a portion of a flow of fluid inthe cooling loop to bypass the fuel-tank heat exchanger.

In some embodiments, the cooling loop may include modulating valves. Themodulating valves may be connected to the fluid conduit and configuredto vary the flow of fluid in the cooling loop through the air-streamheat exchanger.

In some embodiments, the second heat source may be at least one of abattery, an avionic system, and a directed energy weapon. In someembodiments, the second heat source may be a battery. In someembodiments, the second heat source may include a battery, an avionicsystem, and a directed energy weapon.

In some embodiments, the thermal energy system may further include acontrol system. The control system may be configured to selectively varya flow of the fluid in the cooling loop through the air-stream heatexchanger and the fuel-tank heat exchanger to maintain a temperature ofthe first heat source below a predetermined heat load temperature.

In some embodiments, the control system may include a plurality ofvalves and a controller. The plurality of valves may be connected to thefluid. The controller may be connected to the plurality of valves.

In some embodiments, the controller may be configured to determine anamount of heat transferred between the fluid and the fuel in the firstfuel tank. The controller may be configured to operate the plurality ofvalves to selectively vary the flow of fluid in the cooling loop throughthe fuel-tank heat exchanger in response to the amount of heattransferred being different from a predetermined heat rejectionschedule.

In some embodiments, the controller may be configured to operate theplurality of valves to at least partially block the fluid from flowingthrough the fuel-tank heat exchanger in response to the controllerreceiving a signal indicative of the temperature of the fuel in thefirst fuel tank being equal to or above a first fuel tank predeterminedthreshold temperature. The first fuel tank predetermined thresholdtemperature may be greater than a predetermined maximum allowable fueltemperature.

In some embodiments, the controller may be configured to operate theplurality of valves to allow the fluid to flow through the fuel-tankheat exchanger in response to the controller receiving i) a signalindicative of a temperature of the fuel in the first fuel tank beingequal to or above a first fuel tank predetermined threshold temperatureand ii) a signal indicative of a temperature of the cooling fluid beingless than a predetermined maximum allowable tank temperature. Thecontrol system may be configured to vary the predetermined maximumallowable tank temperature throughout a flight cycle of the aircraft.

In some embodiments, the thermal energy system may further include acontrol system. The control system may include a plurality of valves anda controller. The plurality of valves may be connected to the fluidconduit. The controller may be connected to the plurality of valves.

In some embodiments, the controller may be configured to determine atemperature of the fluid in the cooling loop upstream of an accumulatorincluded in the cooling loop. The controller may be configured tooperate the plurality of valves to selectively vary the flow of fluid inthe cooling loop through the air-stream heat exchanger and through thefuel-tank heat exchanger in response to the temperature of the fluid inthe cooling loop upstream of the accumulator being different from asensor target temperature.

In some embodiments, the cooler may be a ram air cooler. The duct may beconfigured to receive air from atmosphere around the aircraft duringforward movement of the aircraft relative to ground. In someembodiments, the duct may be free of any air mover.

According to another aspect of the present disclosure, a method mayinclude providing a thermal energy system for use with an aircraft. Thethermal energy system may include a cooling loop, a cooler, and athermal-storage fuel system.

In some embodiments, the cooling loop may have a fluid conduit, a pump,and a compressor. The pump may be configured to move fluid through thefluid conduit.

In some embodiments, the cooler may include a duct and an air-streamheat exchanger. The duct may be configured to conduct air through theduct. The air-stream heat exchanger may be located in the duct and inthermal communication with the fluid in the cooling loop to transferheat between the cooling loop and the air conducted through the duct.

In some embodiments, the thermal-storage fuel system may include a firstfuel tank and a fuel-tank heat exchanger. The first fuel tank may beconfigured to store fuel therein. The fuel-tank heat exchanger may be inthermal communication with the fluid in the cooling loop.

In some embodiments, the method further includes conducting the fluidthrough the cooling loop to transfer heat between a first heat sourceand the fluid in the cooling loop and conducting the fluid through thecooling loop to transfer heat between a second heat source and the fluidin the cooling loop. The method may further include conducting the fluidin the cooling loop to the air-stream heat exchanger to transfer heatbetween the fluid in the cooling loop and the air passing through theduct and conducting the fluid in the cooling loop to the fuel-tank heatexchanger to transfer heat between the fluid in the cooling loop andfuel stored in the first fuel tank.

In some embodiments, the method further includes selectively varying aflow of the fluid in the cooling loop through the air-stream heatexchanger and a flow of the fluid through the fuel-tank heat exchangerto maintain a temperature of the first heat source below a predeterminedheat load temperature.

In some embodiments, the thermal-storage fuel system may further includea second fuel tank and an engine fuel unit. The second fuel tank may bein fluid communication with the first fuel tank. The engine-fuel unitmay be configured to receive fuel from the second fuel tank and deliverthe fuel to an engine. In some embodiments, the engine-fuel unit may bein thermal communication with the cooling loop.

In some embodiments, the method may further include conducting the fluidin the cooling loop to the engine-fuel unit before conducting the fluidto the air-stream heat exchanger to transfer heat between the fluid inthe cooling loop and the fuel in the engine-fuel unit.

In some embodiments, the engine-fuel unit may include a valve system andan engine-fuel heat exchanger. The valve system may be in fluidcommunication with the second fuel tank. The engine-fuel heat exchangermay be in thermal communication with the valve system and the coolingloop to transfer heat between the fluid in the cooling loop and the fuelfrom the second fuel tank. In some embodiments, the valve system may beconfigured vary a flow of fuel through the engine-fuel heat exchanger todeliver fuel to the engine at an engine-fuel unit predeterminedthreshold temperature.

These and other features of the present disclosure will become moreapparent from the following description of the illustrative embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is elevation view of an aircraft that includes a gas turbineengine and a thermal energy system connected with the gas turbine engineto manage the temperatures of the electric components of the aircraftand the gas turbine engine;

FIG. 2 is a diagrammatic view of the thermal energy system included inthe aircraft of FIG. 1 showing the thermal energy system includes acooling loop configured to move fluid through system to transfer heatfrom a heat source to the fluid in the fluid conduit to cool the heatsource, a ram air cooler configured to transfer heat between the fluidin the cooling loop to air passing through the ram air cooler, athermal-storage fuel system configured to transfer heat between thefluid in the cooling loop and fuel stored in a first fuel tank;

FIG. 3 is a diagrammatic view similar to FIG. 2 showing the thermalenergy system further includes a controller coupled to a valve connectedto the fluid conduit in the cooling loop to selectively cause the flowof fluid to bypass the fuel-tank heat exchanger;

FIG. 4 is a diagrammatic view of another embodiment of a thermal energysystem adapted for use in the aircraft of FIG. 1 showing the thermalenergy system includes a cooling loop, a ram air cooler, athermal-storage fuel system configured to transfer heat between thefluid in the cooling loop and fuel stored in a first fuel tank, and anengine-fuel unit configured to transfer heat between the fluid in thecooling loop and fuel from a second fuel tank before being delivered tothe gas turbine engine;

FIG. 5 is a diagrammatic view of another embodiment of a thermal energysystem adapted for use in the aircraft of FIG. 1 showing the thermalenergy system includes a cooling loop configured to move oil throughsystem to transfer heat from a heat source to the oil in the fluidconduit to cool the heat source, a ram air cooler, a thermal-storagefuel system configured to transfer heat between the oil in the coolingloop and fuel stored in a first fuel tank, and an engine-fuel unitconfigured to transfer heat from the oil in the cooling loop to the fuelstored in a second fuel tank before being delivered to the gas turbineengine;

FIG. 6 is a diagrammatic view of another embodiment of a thermal energysystem adapted for use in the aircraft of FIG. 1 showing the thermalenergy system includes a cooling loop, a ram air cooler, athermal-storage fuel system, and a low temperature heat load in thermalcommunication with the fluid conduit for transferring heat to the fluidin the fluid conduit;

FIG. 7 is a diagrammatic view similar to FIG. 6 showing the thermalenergy system further includes a control system having a plurality ofvalves connected to the fluid conduit and a controller connected to theplurality of valves to operate the plurality of valves and control theflow of fluid through the thermal-storage fuel system; and

FIG. 8 is a diagrammatic view of another embodiment of a thermal energysystem adapted for use in the aircraft of FIG. 1 showing the thermalenergy system includes a cooling loop, a ram air cooler, athermal-storage fuel system, and a valve connected to the fluid conduitand configured to selectively cause the flow of fluid to bypass acompressor downstream of a low temperature heat load in thermalcommunication with the fluid conduit for transferring heat to the fluidin the fluid conduit.

DETAILED DESCRIPTION OF THE DRAWINGS

For the purposes of promoting an understanding of the principles of thedisclosure, reference will now be made to a number of illustrativeembodiments illustrated in the drawings and specific language will beused to describe the same.

A thermal energy system 12 according to the present disclosure isadapted for use with an aircraft 10 such as the aircraft 10 shown inFIG. 1. The thermal energy system 12 is configured to managetemperatures of components of the aircraft 10, such as components of agas turbine engine 14, electric batteries, motor-generators, and/oroil/hydraulic heat loads 16 of the aircraft 10.

The thermal energy system 12 includes a cooling loop 20, a cooler 22,and a thermal-storage fuel system 24 as shown in FIGS. 2 and 3. Thecooling loop 20 has a fluid conduit 30 and a pump 32 configured to movefluid through the fluid conduit 30 to transfer heat from a heat source16 to the fluid thereby cooling the heat source 16. The cooler 22includes a duct 34 configured to conduct air therethrough and a heatexchanger 36 located in the duct 34. The air-stream heat exchanger 36 isin thermal communication with the fluid in the cooling loop 20 totransfer heat between the cooling loop 20 and the air conducted throughthe duct 34. The thermal-storage fuel system 24 is configured totransfer heat between the fluid in the cooling loop 20 and fuel storedin the thermal-storage fuel system 24. As such, one or both of thecooler 22 and the thermal-storage fuel system 24 may transfer heat withthe fluid.

The thermal-storage fuel system 24 includes a first fuel tank 44, asecond fuel tank 46, and a fuel-tank heat exchanger 48 as shown in FIGS.2 and 3. Both the first fuel tank 44 and second fuel tank 46 areconfigured to store fuel therein. The second fuel tank 46 is in fluidcommunication with the first fuel tank 44 through the valves 52, 53 totransfer fuel selectively therebetween. The fuel-tank heat exchanger 48is in thermal communication with the fluid conduit 30 and is configuredto transfer heat between the fluid in the cooling loop 20 and fuelstored in the first fuel tank 44. In the illustrative embodiment, thethermal-storage fuel system 24 includes a fuel pump 47 configured topump fuel through the fuel-tank heat exchanger 48.

In some embodiments, the cooler 22 may be a ram air cooler 22 and theduct 34 is configured to receive air from the atmosphere 18 around theaircraft 10 during forward movement of the aircraft 10 relative to theground 19 and a ram-air heat exchanger 36 located in the duct 34. Theduct 34 may be free of any air movers (i.e. blades, fans, blowers,etc.). Aircraft may often use ram air to cool heat loads of theaircraft. Such thermal energy systems rely on ram-air heat exchangers toreject the waste heat produced by power electronic devices or electricmachines such as generators, motors, and/or engines. However, thetemperature difference between the component being cooled and thetemperature of the air flowing through the ram-air heat exchanger maycause design challenges.

For example, when the aircraft 10 is stationary or taxiing, little or noram pressure is created to force air through the ram-air heat exchanger36. As a result, there is little or no air to cool the coolant in thecooling loop 20. Therefore, some typical aircraft may include a blowerto force air through the ram-air heat exchanger, which may addadditional weight and volume to the system. The system of the presentdisclosure does not include or use a blower.

The thermal energy system 12 of the present disclosure includes thethermal-storage fuel system 24 to eliminate the blower and reduce a sizeof the duct 34 and heat exchanger compared to typical systems therebyincreasing efficiency of the thermal energy system 12 and reducing sizeand weight of the aircraft. The thermal-storage fuel system 24 uses theon-board fuel in the first fuel tank 44 as a heat sink for the heatloads 16 produced early in the flight cycle. In the illustrativeembodiment, the cooling loop 20 is structured to conduct the fluidheated from the heat source 16 through the air-stream heat exchanger 36followed by the fuel-tank heat exchanger 48 after the air-stream heatexchanger 36.

In this way, the fuel in the first and second fuel tanks 44, 46 is nottransported to the heat loads 16, but rather, the fluid is brought tothe fuel to transfer the heat with the fuel. Transferring heat with thefuel in the first fuel tank 44 minimizes safety risks, and heating thefuel prior to burning in the engine 14 may provide additional benefits.

In the illustrative embodiment, the thermal energy system 12 furtherincludes a control system 26 as shown in FIGS. 2 and 3. The controlsystem 26 is configured to manage the temperature of the heat load 16 sothat the heat load 16 does not exceed threshold temperatures, i.e. apredetermined heat load temperature. In some embodiments, the heat load16 may comprise of multiple heat loads 16 that may be arranged inseries, parallel, or a combination. The control system 26 is alsoconfigured to manage the flow of fluid in the cooling loop 20 so thatcooling of the heat load 16 is provided in the most efficient way. Inother words, the control system 26 is configured to manage the flow offluid so as to minimize the fuel burned to complete the flight cycle.The thermal-storage fuel system 24 adds the capability to reduce theamount of heat rejected to the environment and to determine the mostefficient time to reject the heat to the environment.

The control system 26 includes valves 52, 53, 54, 55, 57, sensors 62,63, 64, 66, 67, and a controller 68 as shown in FIGS. 2 and 3. Thevalves 52, 53 are configured to transfer fuel between the fuel tanks 44,46 to cool the fuel in the first fuel tank 44 or heat the fuel in thesecond fuel tank 46. The valve 54 is connected to the fluid conduit 30downstream of the air-stream heat exchanger 36 and configured toselectively cause a flow of fluid to bypass the fuel-tank heat exchanger48. The valves 55, 57 are configured to vary the flow rate of the fluidin the cooling loop 20 through the air-stream heat exchanger 36. Thesensor 62 is configured to measure the temperature of the fuel at theinlet of the fuel pump 47, while the sensor 63 is configured to measurethe temperature of the fuel at the outlet of the fuel pump 47. Thesensor 64 is configured to measure the temperature of the fluid in thefuel tank 46. The sensor 66 is configured to measure the temperature ofthe fluid in the cooling loop 20 directly upstream of the fuel-tank heatexchanger 48 as it enters the fuel-tank heat exchanger 48. The sensor 67is configured to measure the temperature of the fluid in the coolingloop 20 as it returns to an accumulator 38 included in the cooling loop20. In some embodiments, where the cooling loop 20 is a two-phase flowsystem, the sensor 67 may be a pressure gauge.

The controller 68 is configured to manage the cooling of the heat loads16 as well as the use of the thermal-storage fuel system 24. Thecontroller 68 is configured to direct the valves 54, 55, 57 to vary theflow of the fluid in the cooling loop 20. The controller 68 is alsoconfigured to control the fuel pump flow rate by the fuel pump 47included in the thermal-storage fuel system 24 through the fuel-tankheat exchanger 48. The controller 68 controls the valves 54, 55, 57 andthe fuel pump 47 so as to maintain the fluid in the cooling loop 20 at apredetermined temperature.

The controller 68 is configured to maximize the fuel savings of theaircraft 10 based on several factors, such as the type of day (hot orcold), the temperature along the flight path. In the illustrativeembodiment, the controller 68 is configured to gather the state of thethermal energy system 12 prior to the flight as well as expectedconditions along the flight path. Before take-off, the controller 68 isconfigured to assess the amount of fuel on board the aircraft 10 in eachof the first fuel tank 44 and the second fuel tank 46, as well astemperature of each tank 44, 46. This establishes the initial heat sinkcapacity on board the aircraft 10.

Additionally, the controller 68 is configured to analyze the anticipatedflight path, including speed, altitude, air conditions (temperature,wind speed) to determine the anticipated change in weight of the fuelover the flight profile and the expected temperature of the air heatsink. The controller 68 is configured to use flight path conditions todetermine when heat should be added to the first fuel tank 44 and whenit would be best to remove heat from the first fuel tank 44 to minimizefuel burn.

The control system 26 may be configured with model-based controls thatrun simulations prior to the flight. These simulations may be achievedthrough the use of artificial intelligence, general rules, or physicsbased calculations. The control system 26 may be configured to generatea schedule of heat to be rejected to the first fuel tank 44 as afunction of location in the flight path.

In some embodiments, the controller 68 is configured to transfer as muchheat into the fuel as possible, while not exceeding any system thermallimits. In this way, a greater percentage of the waste heat to beremoved may be returned to the engine cycle to produce useful work.Therefore, on a cold day, the amount of heat transferred to the firstfuel tank 44 may be much larger than on a hot day.

In addition, the controller 68 may be configured to minimize drag forceson the aircraft 10. With a variable air stream 35 passing through theair-stream heat exchanger 36, the cooling in the air-stream heatexchanger 36 may be timed to minimize fuel burn over the mission.

Using this information, a pre-flight fuel tank heat rejection schedulemay be generated to control the flow of fluid in the cooling loop 20 tothe thermal-storage fuel system 24. The schedule also includesre-cooling of the fuel tanks 44, 46 in preparation for being able toburn the fuel stored in the first fuel tank 44 and to also providecooling during descent and taxiing after landing. This could requiresignificant cooling of the fuel tanks 44, 46 while still at altitude.

In addition, the control system 26 may be configured to mix fuel betweenthe first and second fuel tanks 44, 46 to maximize the use of the fuelthermal storage. Additionally, the pre-flight schedules may be updatedduring the flight.

Once the pre-flight schedules are established, the aircraft 10 maytake-off. As the flight proceeds, the temperature of the fuel in thefirst fuel tank 44 is measured by the sensors 62, 63. The upstreamsensor 62 measures the temperature of the fuel flowing through the fuelpump 47 and the downstream sensor 63 measures the temperature of thefuel exiting the fuel-tank heat exchanger 48. The controller 68 isconfigured to determine the temperature rise on the fuel stream goingthrough the fuel-tank heat exchanger 48 based on the temperaturemeasurements by the sensors 62, 63. The controller 68 is also configuredto measure or calculate the fuel flow rate based on operating conditionsof the fuel pump 47.

In this way, the controller 68 measures the actual heat rejection orheat absorption between the fluid and the fuel in the first fuel tank44. The control system 26 is configured to match the actual heattransfer between the fluid and the fuel in the first fuel tank 44 withthe heat rejection schedule.

To match the actual heat transfer between the fluid and the fuel in thefirst fuel tank 44, the controller 68 may direct the valves 55, 57 tovary the flow of fluid through the air-stream heat exchanger 36. Thecontroller 68 may also direct the valve 54 to vary the flow of fluidthrough the fuel-tank heat exchanger 48. Alternatively, the controller68 may be configured to vary the fuel flow rate through the fuel-tankheat exchanger 48 by changing fuel pump speed of the fuel pump 47.

In some embodiments, the full heat load 16 may not be able to berejected to the fuel in the first fuel tank 44. In that case, theremaining heat load 16 may be rejected through the air-stream heatexchanger 36.

The amount of heat to be rejected through the air-stream heat exchanger36 may be set to satisfy the temperature requirement measured by sensors66, 67. The amount of heat to be rejected through the air-stream heatexchanger 36 may also be driven by the temperature of the fuel in thefirst fuel tank 44. If the fuel in the first fuel tank 44 is approachingthermal limits of the first fuel tank 44, the flow of cooling fluid tothe first fuel tank 44 may be used to cool the fuel in the first tank44.

In some embodiments, the temperature of the fuel in the first fuel tank44 may drive the exit temperature from the air-stream heat exchanger 36measured by the sensor 66. In such embodiments in which heat is to beabsorbed from the fuel in the first fuel tank 44, the temperaturemeasured by the sensor 66 should be lower than the temperature in thefirst fuel tank 44 and may be controlled to a target temperature toachieve the desired heat absorption from the first fuel tank 44.

In some embodiments, the controller 68 is configured to vary the amountof air flowing through the air-stream heat exchanger 36 in the duct 34.Reducing the airflow rate may also reduce the drag on the aircraft 10.

In some embodiments, the controller 68 is configured to direct themodulating valves 55, 57 to vary the coolant flow rate through theair-stream heat exchanger 36. In other embodiments, one of themodulating valves 55 may be replaced with a fixed flow restriction. Thesensor 66 and/or the sensor 67 is configured to measure the temperatureof the fluid to enable the controller 68 to better manage the heatrejection from the air-stream heat exchanger 36.

In some embodiments, the controller 68 is configured to control the flowof the fluid in the cooling loop 20 through the fuel-tank heat exchanger48 based on a temperature measured by the sensor 62. The controller 68may direct the valve 54 to allow the fluid in the cooling loop 20through the fuel-tank heat exchanger 48 in response to the temperatureof the fuel in the first fuel tank 44 being below a first fuel tankpredetermined threshold temperature. The controller 68 may direct thevalve 54 to fully, or partially block the fluid in the cooling loop 20through the fuel-tank heat exchanger 48 in response to the temperatureof the fuel in the first fuel tank 44 being above the first fuel tankpredetermined threshold temperature.

In some embodiments, the first fuel tank predetermined thresholdtemperature may be the maximum allowable tank temperature of the firstfuel tank 44. In some embodiments, the first fuel tank predeterminedthreshold temperature may be the maximum allowable fuel temperature atwhich the fuel can be safely supplied to the engine 14. In someembodiments, the first fuel tank predetermined threshold temperature maybe changed throughout the flight mission.

In typical fuel systems, as fuel is passed to the engine 14, theaircraft 10 may add heat to the fuel. The engine 14 may further heat thefuel before it is burned in the combustor. Often, the engine 14 may havea maximum allowable fuel temperature at which the fuel may be suppliedto the engine 14 that is far lower than the maximum temperature that thefuel will be when it is burned. This temperature limit may be used toensure the fuel entering the engine 14 is cold enough to cool engineheat loads before burned in the combustor.

In some embodiments, the first fuel tank 44 may configured to allow thefuel in the first fuel tank 44 to be heated to the maximum temperatureof the fluid in the cooling loop 20. In other embodiments, the firstfuel tank 44 may be configured to allow the fuel in the first fuel tank44 to be heated to some other temperature limit that is above themaximum allowable fuel temperature that may be delivered to the engine.The second fuel tank 46 stores fuel for the engine 14 at a temperatureequal to or less than the maximum allowable fuel temperature that may bedelivered to the engine 14. The second fuel tank is therefore configuredto provide the fuel at the maximum allowable fuel temperature of theengine 14.

If the temperature of the fuel in the first fuel tank 44 is approachingthe first fuel tank predetermined threshold temperature, one of thevalves 52, 53 may allow some fuel to transfer between the fuel tanks 44,46 to cool the fuel in the first fuel tank 44 or heat the fuel in thesecond fuel tank 46. As such, the controller 68 may determine if thevalve 52 may transfer fuel between the fuel tanks 44, 46 based at leastpartially on the temperature measured by the sensors 62, 63, 64.

The valves 52, 53 provide fluid communication between the first andsecond fuel tanks 44, 46. In some embodiments, the first fuel tank 44may have an inlet valve 53 and outlet valve 52 that are fluidlyconnected to the second fuel tank 46. The hot fuel in the first fueltank 44 exits the first fuel tank 44 through the outlet 52 to the secondfuel tank 46, while the relatively cooler fuel enters the first fueltank 44 through the inlet 53 from the second fuel tank 46. The exchangerof the fuel between the first and second fuel tanks 44, 46 may beassisted with the fuel pump 47 through various piping arrangements andvalves.

The thermal-storage fuel system 24 may further include another pump influid communication between the tanks 44, 46. The second pump may beconfigured to move the fuel between the tanks 44, 46.

In other embodiments, the system 12 may include a single fuel tank. Thesingle fuel tank may be coupled to engine 14 to deliver the fuel to theengine 14. In such embodiments, the first fuel tank predeterminedthreshold temperature is a single fuel tank predetermined thresholdtemperature. In such embodiments, the single fuel tank predeterminedthreshold temperature is the maximum allowable fuel temperature at whichthe fuel can be safely passed to the engine 14.

In some embodiments, the controller 68 may also be configured to controlthe flow of the fluid in the cooling loop 20 through the fuel-tank heatexchanger 48 based on the temperature measured by the sensors 62, 63,66, 67. The controller 68 may be configured to direct the valve 54 tocontrol the fluid in the cooling loop 20 through the fuel-tank heatexchanger 48 in response to a temperature measured by at least one ofthe sensors 62, 63, 66, 67.

If the temperature of the fluid directly upstream of the fuel-tank heatexchanger 48 measured by the sensor 66 is below the first fuel tankpredetermined threshold temperature, while the temperature of the fuelin the first fuel tank 44 is equal to or exceeds the first fuel tankpredetermined threshold temperature, the controller 68 may allow theflow of fluid to the fuel-tank heat exchanger 48. Conversely, if thetemperature of the fluid directly upstream of the fuel-tank heatexchanger 48 is above the first fuel tank predetermined thresholdtemperature, while the temperature of the fuel in the first fuel tank 44is equal to or exceeds the first fuel tank predetermined thresholdtemperature, the controller 68 may fully, or partially block the flow offluid to the fuel-tank heat exchanger 48.

In other embodiments, if the temperature of the fluid returning to theaccumulator 38 measured by the sensor 67 is below a sensor targettemperature, and the first fuel tank temperature is greater than thesensor target temperature, the controller 68 may direct the valve 54 toallow the flow of fluid to the fuel-tank heat exchanger 48. Conversely,if the temperature measured by the sensor 67 is above the sensor targettemperature, the controller 68 may direct the valve 54 to fully, orpartially block the flow of fluid to the fuel-tank heat exchanger 48.

During use of the thermal energy system 12 in the aircraft 10, the fluidin the fluid conduit 30 is pumped from the accumulator 38 to the heatsource 16 as indicted by arrow 80. The pump 32 moves the fluid throughthe fluid conduit 30 to transfer heat from the heat source 16 to thefluid to cool the heat source 16 as indicted by arrow 82. Moving fromthe heat source 16, the flow of fluid in the fluid conduit 30 movesthrough the air-stream heat exchanger 36 to transfer heat between thefluid in the cooling loop 20 and the air passing through the duct 34 ofthe cooler 22 as indicated by arrow 84. The modulating valves 55, 57 areconfigured to vary the flow of fluid through the air-stream heatexchanger 36 based at least in part on the temperature measured by thesensor 62, 63, 64, 66, 67.

As the fluid in the fluid conduit 30 exits the air-stream heat exchanger36 as indicated by arrow 86, the valve 54 controls the flow of fluid tothe thermal-storage fuel system 24. The valve 54 is connected to thefluid conduit 30 downstream of the air-stream heat exchanger 36 toselectively cause the flow of fluid to fully, or partially bypass thefuel-tank heat exchanger 48 based at least in part on the temperaturemeasured by the sensors 62, 63, 64, 66, 67.

The controller 68 may be configured to direct the valve 54 to allow theflow of the fluid in the fluid conduit 30 through the fuel-tank heatexchanger 48 based on the temperature measured by one or all of thesensors 62, 63, 64 as shown in FIG. 2. The valve 54 allows the fluid inthe fluid conduit 30 to flow to the fuel-tank heat exchanger 48 asindicated by arrow 88 in FIG. 2. The valve 54 allows the fluid to flowthrough the fuel-tank heat exchanger 48 in response to the temperatureof the fuel in the first fuel tank 44 being below the first fuel tankpredetermined threshold temperature.

Additionally, the valve 54 may also allow the fluid in the fluid conduit30 to flow to the fuel-tank heat exchanger 48 in response to thetemperature of the fluid in the cooling loop 20 directly upstream of thefuel-tank heat exchanger 48. If the temperature of the fluid directlyupstream of the fuel-tank heat exchanger 48 is below the first fuel tankpredetermined threshold temperature, while the temperature of the fuelin the first fuel tank 44 is equal to or exceeds the first fuel tankpredetermined threshold temperature, the valve 54 allows the flow offluid to the fuel-tank heat exchanger 48. As the fluid in the fluidconduit 30 exits the fuel-tank heat exchanger 48, the flow of fluidreturns to the accumulator 38 as indicated by arrow 90 in FIG. 2.

During taxiing or take-off of the aircraft 10, the air flowing throughthe ram air cooler 22 may be about the same temperature of the fluid inthe fluid conduit 30. As such, the heat transfer between the fluid andthe air-stream heat exchanger 36 is low such that the heat of the fluidin the cooling loop 20 is not sufficiently removed. To expel heat formthe system 12, the valve 54 may allow the fluid in the fluid conduit 30to flow to the fuel-tank heat exchanger 48 to transfer heat from thefluid in the cooling loop 20 to the fuel in the first fuel tank 44 asshown in FIG. 2.

In the illustrative embodiment, as the valve 54 allows fluid in thefluid conduit 30 to flow to the fuel-tank heat exchanger 48, the valve54 may fully, or partially, block flow from bypassing the fuel-tank heatexchanger 48 as suggested by the dotted line 92 in FIG. 2. As such,little to no fluid is flowing in the fluid conduit 30 in the region ofthe dotted line 92 as shown in FIG. 2.

In other embodiments, the valve 54 may partially allow fluid in thefluid conduit 30 to flow through the fuel-tank heat exchanger 48. Assuch, some fluid may be flowing to the fuel-tank heat exchanger 48 andto the accumulator 38. In such embodiments, the lines 88, 90, 92 wouldbe solid to show the flow of fluid in fluid conduit 30 to each of thecomponents 48, 38.

In the illustrative embodiment, the ram air cooler 22 may be bypassedduring taxiing or take off to reduce the amount of drag created by theduct 34. As the majority, if not all, of the heat from the heat source16 is transferred through the fuel-tank heat exchanger 48; the ram aircooler 22 may be closed off to block air flowing through the ram aircooler 22, to minimize drag.

In the illustrative embodiment, the ram air cooler 22 further includes avariable NACA inlet 33 as shown in FIGS. 2 and 3. The variable NACAinlet 33 will reduce the amount of drag created from the duct 34 whenthe ram air cooler 22 is not being used. The variable inlet 33 variesthe amount of air from the atmosphere 18 through the air-stream heatexchanger 36 as indicated by arrow 35.

As the fluid in the fluid conduit 30 heats the fuel in the first fueltank 44, the temperature of the fuel approaches the first fuel tankpredetermined threshold temperature. However, at higher altitudes, theair is cooler in temperature and therefore the air flowing through theram air cooler 22 is viable for transferring heat from the fluid to theair. Therefore, as the aircraft 10 increases in altitude and/or speed,the air-stream heat exchanger 36 may begin to transfer more heat fromthe fluid in the cooling loop 20 to the atmosphere air flowing throughthe duct 34.

To manage the temperature of the fuel in the first fuel tank 44 and thefluid in the fluid conduit 30, the controller 68 directs the valve 54 tofully, or partially block the flow of the fluid through the fuel-tankheat exchanger 48 based on the temperature measured by the sensors 62,63, 64, 66, 67 as shown in FIG. 3. If the temperature of the fluidmeasured by the sensor 66 is above the first fuel tank predeterminedthreshold temperature, while the temperature of the fuel in the firstfuel tank 44 is equal to or exceeds the first fuel tank predeterminedthreshold temperature, the controller 68 may fully, or partially blockthe flow of fluid to the fuel-tank heat exchanger 48. Dotted arrows 88,90, indicate the blocked flow of fluid to the fuel-tank heat exchanger48.

If the valve 54 blocks the fluid in the cooling loop 20 from flowing tothe fuel-tank heat exchanger 48, the fluid flows back to the accumulator38 as suggested by the arrow 92 in FIG. 3. However, as the air-streamheat exchanger 36 begins to transfer more heat to the air, the fluid inthe cooling loop 20 may be cool enough to transfer, or absorb heat fromthe fuel in the first fuel tank 44.

As such, the valve 54 allows the flow of the fluid in the fluid conduit30 through the fuel-tank heat exchanger 48 if the temperature of thefluid measured by the sensor 66 is below the first fuel tankpredetermined threshold temperature, while the temperature of the fuelin the first fuel tank 44 is equal to or exceeds the first fuel tankpredetermined threshold temperature. In this way, the temperature of thefuel in the first fuel tank 44 is reduced in preparation to act as aheat sink later in the flight, as the aircraft 10 descends into warmerair. In such environments, the air-stream heat exchanger 36 may not beable to reject as much heat to the air.

A method of cooling heat loads on an aircraft 10 may include severalsteps. The method includes conducting a cooling fluid through thecooling loop 20 to transfer heat between the heat source 16 and thefluid in the cooling loop 20, conducting the fluid in the cooling loop20 to the air-stream heat exchanger 36 to transfer heat between thefluid in the cooling loop 20 and the air passing through the duct 34,and conducting the fluid in the cooling loop 20 to the fuel-tank heatexchanger 48 to transfer heat between the fluid in the cooling loop 20and fuel stored in the first fuel tank 44. In this way, the fuel in thefirst and second fuel tanks 44, 46 is not transported to the heat loads16, but rather, the fluid is brought to the fuel to transfer the heatwith the fuel. Transferring heat with the fluid to the fuel in the firstfuel tank 44 minimizes safety risks, and heating the fuel prior toburning in the engine 14 may provide additional benefits.

In the illustrative embodiment, the method further includes selectivelyvarying a flow of the fluid in the cooling loop 20 through theair-stream heat exchanger 36 and a flow of the fluid through thefuel-tank heat exchanger 48. Varying the flows of the fluid through theair-stream heat exchanger 36 and the fuel-tank heat exchanger 48maintains a temperature of the heat source 16 below a predetermined heatload temperature.

In the illustrative embodiment, the method further includes measuringthe heat transfer between the fluid and the fuel in the first fuel tank44 and comparing the heat transfer measured to a predetermined heatrejection schedule. Based on the comparison, the flow of fluid in thecooling loop 20 through the fuel-tank heat exchanger 48 is selectivelyvaried in response to the heat transfer measured being different fromthe predetermined heat rejection schedule.

In the illustrative embodiment, the method further includes measuringthe temperature of the fluid in the cooling loop 20 upstream of theaccumulator 38 included in the cooling loop 20 using the sensor 67. Thetemperature measured is then compared to the sensor target temperatureand the flow of fluid in the cooling loop 20 is selectively variedthrough the air-stream heat exchanger 36 and fuel tank heat exchanger 48in response to the temperature measured being different from the sensortarget temperature.

Another embodiment of a thermal energy system 212 in accordance with thepresent disclosure is shown in FIG. 4. The thermal energy system 212 issubstantially similar to the thermal energy system 12 shown in FIGS. 1-3and described herein. Accordingly, similar reference numbers in the 200series indicate features that are common between the thermal energysystem 212 and the thermal energy system 12. The description of thethermal energy system 12 is incorporated by reference to apply to thethermal energy system 212, except in instances when it conflicts withthe specific description and the drawings of the thermal energy system212.

The thermal energy system 212 includes a cooling loop 220, a cooler 222,a thermal-storage fuel system 224, and a control system 226 as shown inFIG. 4. The cooling loop 220 has a fluid conduit 230 and a pump 232configured to move fluid through the fluid conduit 230 to transfer heatfrom the heat source 216 to the fluid thereby cooling the heat source216. The cooler 222 includes a duct 234 configured to conduct airtherethrough and an air-stream heat exchanger 236 located in the duct234. The air-stream heat exchanger 236 is in thermal communication withthe fluid conduit 230 to transfer heat between the fluid in the coolingloop 220 and the air passing through the duct 234. The thermal-storagefuel system 224 is configured to transfer heat between the fluid in thecooling loop 220 and fuel stored in the thermal-storage fuel system 224.

The control system 226 is configured to control the flow of fluidthrough the cooling loop 220 to manage the cooling of the heat loads 216as well as manage the use of the thermal-storage fuel system 224.

The thermal-storage fuel system 224 includes fuel tanks 244, 246, afuel-tank heat exchanger 248, and an engine-fuel unit 250 as shown inFIG. 4. Both the first fuel tank 244 and second fuel tank 246 areconfigured to store fuel therein, while the second fuel tank 246 is influid communication with the first fuel tank 244. The fuel-tank heatexchanger 248 is in thermal communication with the fluid conduit 230 andis configured to transfer heat between the fluid in the cooling loop 220and fuel stored in the first fuel tank 244. The engine-fuel unit 250 isconfigured to receive fuel from the second fuel tank 246 and deliver thefuel to the engine 214. In the illustrative embodiment, the engine-fuelunit 250 is in thermal communication with the cooling loop 220 totransfer heat between the fluid in the cooling loop 220 and the fuel inthe engine-fuel unit 250.

The engine-fuel unit 250 includes a mix valve 274, conduits 276, 278,and an engine-fuel heat exchanger 279 as shown in FIG. 4. The firstconduit 276 is in fluid communication between the mix valve 274 and thesecond fuel tank 246 to deliver fuel having a first temperature to themix valve 274. The second conduit 278 is in fluid communication betweenthe mix valve 274 and the second fuel tank 246. The engine-fuel heatexchanger 279 in thermal communication with the second conduit 278 tocause the second conduit 278 to deliver fuel having a second temperatureto the mix valve 274.

In the illustrative embodiment, the mix valve 274 and the conduits 276,278 form a valve system in fluid communication with the second fuel tank246 and the engine-fuel heat exchanger 279. The valve system, includingthe mix valve 274 and the conduits 276, 278, is configured to vary aflow of fuel through the engine-fuel heat exchanger 279 to deliver fuelto the engine 214 at an engine-fuel unit predetermined thresholdtemperature.

In the illustrative embodiment, the engine-fuel unit predeterminedthreshold temperature is the desired temperature for the fuel deliveredto the engine 214 from the valve system. In some embodiments, theengine-fuel unit predetermined threshold temperature may be equal to amaximum allowable fuel temperature. The maximum allowable fueltemperature is the maximum temperature that the fuel may be when it ispassed to the engine 214.

In some embodiments, the mix valve 274 is configured to control the flowof fuel to the engine-fuel heat exchanger 279 to match the maximumallowable fuel temperature. The maximum allowable fuel temperature mayvary throughout the flight profile or be a single value.

In some embodiments, the mix valve 274 is configured to vary a firstflow rate of fuel from the first conduit 276 and a second flow rate offuel from the second conduit 278. The mix valve 274 varies the first andsecond flow rates for the respective conduits 276, 278 to provide amixed stream of fuel having a third temperature. The third temperaturemay also be equal to or less than the maximum allowable fueltemperature, which is the temperature at which the fuel can be safelysupplied to the engine 214.

The maximum allowable tank temperature of the second fuel tank 246 maybe less than the maximum allowable fuel temperature. In this way, thefuel in the second fuel tank 246 may absorb heat from the heat loads 216and be below the maximum allowable fuel temperature.

In the illustrative embodiments, the maximum allowable tank temperatureof the second fuel tank 246 may be less than the maximum allowable tanktemperature of the first fuel tank 244. The maximum allowable fueltemperature, i.e. the temperature at which the fuel can be safelysupplied to the engine 14, may be equal to or greater than the maximumallowable tank temperature of the second tank 246.

The maximum allowable tank temperature of the first fuel tank 244 may begreater than or equal to the maximum allowable fuel temperature. Thefuel in the first fuel tank 244 may be heated to a temperature greaterthan that of the maximum allowable fuel temperature for the engine 214because the first fuel tank 244 may be cooled during the mission beforethe fuel in the first fuel tank 244 may need to be sent to the engine214.

In some embodiments, the engine-fuel unit predetermined thresholdtemperature may be equal to or greater than the first fuel tankpredetermined threshold temperature. In some embodiments, theengine-fuel unit predetermined threshold temperature may vary throughoutthe flight mission.

Turning again to the control system 226, the control system 226 includesthe valves 254, 255, 257, sensors 262, 263, 264, 266, 267, and acontroller 268 as shown in FIG. 4. The valve 254 is connected to thefluid conduit 230 and configured to selectively cause a flow of fluid tobypass the fuel-tank heat exchanger 248. The valves 255, 257 areconnected to the fluid conduit 230 and configured to selectively varythe flow through the heat exchanger 236. The sensor 262 is configured tomeasure the temperature of the fuel at the inlet of a fuel pump 247,while the sensor 263 is configured to measure the temperature of thefuel at the outlet of the fuel pump 47. The sensor 264 is configured tomeasure the temperature of the fluid in the fuel tank 246. The sensor266 is configured to measure the temperature of the fluid in the coolingloop 20 directly upstream of the fuel-tank heat exchanger 248 as itenters the fuel-tank heat exchanger 248. The sensor 267 is configured tomeasure the temperature of the fluid in the cooling loop 220 as itreturns to an accumulator 238 included in the cooling loop 220.

The controller 268 is configured to manage the cooling of the heat loads216 as well as the use of the thermal-storage fuel system 224. Thecontroller 268 is configured to direct the valves 254, 255, 257 to varythe flow of the fluid in the cooling loop 220. The controller 268 isalso configured to control the fuel pump flow rate by the fuel pump 247included in the thermal-storage fuel system 224 through the fuel-tankheat exchanger 248. The controller 268 controls the valves 254, 255, 257and the fuel pump 247 so as to maintain the fluid in the cooling loop220 at a predetermined temperature.

In the illustrative embodiments, the controller 268 is configured toallow the flow of the fluid in the cooling loop 220 through thefuel-tank heat exchanger 248 based on a temperature measured by thesensor 262. The controller 268 directs the valve 254 to allow the fluidin the cooling loop 220 through the fuel-tank heat exchanger 248 inresponse to a temperature of the fuel in the first fuel tank 244 beingbelow the first fuel tank predetermined threshold temperature.

In the illustrative embodiments, the controller 268 is also configuredto allow the flow of the fluid in the cooling loop 220 through thefuel-tank heat exchanger 248 based on the temperature measured by thesensors 262, 263, 266, 267. The controller 268 may be configured todirect the valve 254 to control the fluid in the cooling loop 220through the fuel-tank heat exchanger 248 in response to a temperaturemeasured by at least one of the sensors 262, 263, 266, 267.

If the temperature of the fluid directly upstream of the fuel-tank heatexchanger 248 is below the first fuel tank predetermined thresholdtemperature, while the temperature of the fuel in the first fuel tank244 is equal to or exceeds the first fuel tank predetermined thresholdtemperature, the controller 268 allows the flow of fluid to thefuel-tank heat exchanger 248. Conversely, if the temperature of thefluid directly upstream of the fuel-tank heat exchanger 248 is above thefirst fuel tank predetermined threshold temperature, while thetemperature of the fuel in the first fuel tank 244 is equal to orexceeds the first fuel tank predetermined threshold temperature, thecontroller 268 may fully, or partially block the flow of fluid to thefuel-tank heat exchanger 248.

In other embodiments, if the temperature of the fluid returning to theaccumulator 238 measured by the sensor 267 is below the sensor targettemperature, the controller 268 may direct the valve 254 to allow theflow of fluid to the fuel-tank heat exchanger 248, if the fuel tanktemperature is higher than the sensor target temperature. Conversely, ifthe temperature measured by the sensor 267 is above the sensor targettemperature, the controller 268 may direct the valve 254 to fully, orpartially block the flow of fluid to the fuel-tank heat exchanger 248 ifthe fuel tank temperature is higher than the sensor temperature.However, if the fuel tank temperature is lower than the sensor targettemperature, valve 254 will pass flow to the fuel tank to providecooling to the fluid.

During use of the thermal energy system 212 in the aircraft 10, thefluid in the fluid conduit 230 is pumped from the accumulator 238included in the cooling loop 220 to the heat source 216 as indicted byarrow 280. The pump 232 moves the fluid through the fluid conduit 230 totransfer heat from the heat source 216 to the fluid to cool the heatsource 216 as indicted by arrow 282. Moving from the heat source 216,the flow of fluid in the fluid conduit 230 moves through the engine-fuelheat exchanger 279 of the engine-fuel unit 250 as indicated by arrow283. The fluid in the fluid conduit 230 is in fluid communication withthe engine-fuel heat exchanger 279 to transfer heat between the fluidand the fuel flowing to the engine 214.

As the fluid in the fluid conduit 230 exits the engine-fuel unit 250,the fluid flows to the ram-air heat exchanger 236 as indicated by arrow284. The fluid flows through the ram-air heat exchanger 236 to transferheat between the fluid in the cooling loop 20 and the air passingthrough the duct 34 of the ram air cooler 22.

As the fluid in the fluid conduit 230 exits the ram-air heat exchanger236 as indicated by arrow 286, the valve 254 controls the flow of fluidto the thermal-storage fuel system 224. The valve 254 is configured toselectively cause the flow of fluid to bypass the fuel-tank heatexchanger 248 based at least in part on the temperature measured by thesensors 262, 263, 264, 266, 267.

The valve 254 may allow the fluid in the fluid conduit 230 to flow tothe fuel-tank heat exchanger 248 in response to the temperature of thefuel in the first fuel tank 244 being below the first fuel tankpredetermined threshold temperature as indicated by arrow 288. The valve254 may also allow the fluid to flow to the fuel-tank heat exchanger 248in response to the temperature of the fluid directly upstream of thefuel-tank heat exchanger 248 being below the first fuel tankpredetermined threshold temperature, while the temperature of the fuelin the first fuel tank 44 is equal to or exceeds the first fuel tankpredetermined threshold temperature.

In the illustrative embodiment, as the valve 254 allows fluid in thefluid conduit 230 to flow to the fuel-tank heat exchanger 248, the valve254 blocks flow from bypassing the fuel-tank heat exchanger 248 assuggested by the dotted line 292 in FIG. 4. As such, no fluid is flowingin the fluid conduit 230 in the region of the dotted line 292. As thefluid in the fluid conduit 230 exits the fuel-tank heat exchanger 248,the flow of fluid returns to the accumulator 238 as show by arrow 290.

As discussed above, the controller 268 is configured to direct the valve254 to fully, or partially block the flow of the fluid in the fluidconduit 230 through the fuel-tank heat exchanger 248 based on thetemperature measured by the sensors 262, 263, 264, 266, 267. If thevalve 254 partially or fully blocks the fluid in the fluid conduit 230from flowing to the fuel-tank heat exchanger 248, the rest or all offluid flows to the accumulator 238. As such, little to no fluid isflowing in the fluid conduit 230 in and out of the fuel-tank heatexchanger 248.

As the fluid in the fluid conduit 230 heats the fuel in the first fueltank 244, the temperature of the fuel approaches the first fuel tankpredetermined threshold temperature. As such, the fuel in the secondfuel tank 246 may be used as an additional heat sink to prevent thetemperature of the fuel in the first fuel tank 244 from exceeding thefirst fuel tank predetermined threshold temperature.

The fluid in the fluid conduit 230 flowing to the engine-fuel heatexchanger 279 heats the fuel in the second conduit 278 to cool the fluidin the fluid conduit 230. The mix valve 274 varies the first flow rateof fuel from the first conduit 276 and the second flow rate of fuel fromthe second conduit 278 to provide the mixed stream of fuel with thethird temperature. In the illustrative embodiments, the controller 268control the flow of fluid to put as much heat into the fuel flowingthrough the engine-fuel heat exchanger 279 without exceeding theengine-fuel unit predetermined threshold temperature.

However, at higher altitudes, the air is cooler in temperature andtherefore the air flowing through the cooler 222 is viable fortransferring heat between the fluid and the air. Therefore, as theaircraft 10 increases in altitude, the ram-air heat exchanger 236 maybegin to transfer more heat between the fluid in the cooling loop 220and the atmosphere air flowing through the duct 234.

As the engine-fuel unit 250 and the ram-air heat exchanger 236 begin totransfer more heat out of the system, the fluid in the cooling loop 220may be cool enough to transfer heat from the fuel in the first fuel tank244. Thus, during flight, the temperature of the first fuel tank 244 islowered below the maximum allowable tank temperature of the first tank244. This cooling process allows the first fuel tank 244 to be able toabsorb more heat from the fluid in the cooling loop 220 later in theflight cycle when cooling the fluid in the fluid conduit 230 may be morechallenging. In this way, the temperature of the fuel in the first fueltank 244 is reduced in preparation to act as a heat sink later in theflight, as the aircraft 10 descends into warmer air. In suchenvironments, the air-stream heat exchanger 236 may not be able toreject as much heat to the air.

In the illustrative embodiment, the fluid in the cooling loop isconducted to the engine-fuel unit before it is conducted to theair-stream heat exchanger. The fluid in the cooling loop is conducted tothe engine-fuel unit before the air-stream heat exchanger to transferheat between the fluid in the cooling loop and the fuel in theengine-fuel unit.

A method of cooling heat loads on the aircraft 10 may include severalsteps and may be substantially similar to the method described inregards to the embodiment shown in FIGS. 1-3. The method furtherincludes varying the flow of fuel through the engine-fuel heat exchanger279 to deliver fuel to the engine 214 at the engine-fuel unitpredetermined threshold temperature. The mix valve 274 varies the firstflow rate of fuel from the first conduit 276 and the second flow rate offuel from the second conduit 278 to provide the mixed stream of fuel.The mixed stream of fuel has the third temperature that is less than orequal to the engine-fuel unit predetermined threshold temperature.

Another embodiment of a thermal energy system 312 in accordance with thepresent disclosure is shown in FIG. 5. The thermal energy system 312 issubstantially similar to the thermal energy system 12 shown in FIGS. 1-3and described herein. Accordingly, similar reference numbers in the 300series indicate features that are common between the thermal energysystem 312 and the thermal energy system 12. The description of thethermal energy system 12 is incorporated by reference to apply to thethermal energy system 312, except in instances when it conflicts withthe specific description and the drawings of the thermal energy system312.

The thermal energy system 312 includes a cooling loop 320, a cooler 322,and a thermal-storage fuel system 324 as shown in FIG. 5. The coolingloop 320 has a fluid conduit 330 and a pump 332 configured to move fluidthrough the fluid conduit 330 to transfer heat from the heat source 316to the fluid in the fluid conduit 330 thereby cooling the heat source316. The cooler 322 includes a duct 334 configured to receive air fromthe atmosphere 18 and an air-stream heat exchanger 336 located in theduct 334. The air-stream heat exchanger 336 is in thermal communicationwith the fluid conduit 330 to transfer heat between the fluid in thecooling loop 320 and the air passing through the duct 334. Thethermal-storage fuel system 324 is configured to transfer heat betweenthe fluid in the cooling loop 320 and fuel stored in the thermal-storagefuel system 324.

The fluid in the illustrative embodiment is oil, which is stored in anoil tank 338 included in the cooling loop 320 as shown in FIG. 5. In theother embodiments, the fluid of the cooling loop 20, 220 is arefrigerant. In the illustrative embodiment of FIG. 5, the oil tank 338replaces the accumulator using oil as the fluid in the cooling loop 320.

The thermal-storage fuel system 324 includes fuel tanks 344, 346, afuel-tank heat exchanger 348, and an engine-fuel unit 350 as shown inFIG. 5. Both the first fuel tank 344 and second fuel tank 346 areconfigured to store fuel therein, while the second fuel tank 346 is influid communication with the first fuel tank 344. The fuel-tank heatexchanger 348 is in thermal communication with the fluid conduit 330 andis configured to transfer heat from the oil in the cooling loop 320 andfuel stored in the first fuel tank 344. The engine-fuel unit 350 isconfigured to receive fuel from the second fuel tank 346 and deliver thefuel to the engine 314. In the illustrative embodiment, the engine-fuelunit 350 is in thermal communication with the cooling loop 320 totransfer heat from the oil in the cooling loop 320 to the fuel in theengine-fuel unit 346.

Another embodiment of a thermal energy system 412 in accordance with thepresent disclosure is shown in FIGS. 6 and 7. The thermal energy system412 is substantially similar to the thermal energy system 12 shown inFIGS. 1-3 and described herein. Accordingly, similar reference numbersin the 400 series indicate features that are common between the thermalenergy system 412 and the thermal energy system 12. The description ofthe thermal energy system 12 is incorporated by reference to apply tothe thermal energy system 412, except in instances when it conflictswith the specific description and the drawings of the thermal energysystem 412.

The thermal energy system 412 includes a cooling loop 420, a cooler 422,a thermal-storage fuel system 424, a control system 426, a first heatsource 416, and a second heat source 428 as shown in FIGS. 6 and 7. Thecooling loop 420 has a fluid conduit 430 and a pump 432 configured tomove fluid through the fluid conduit 430 to transfer heat from the heatsources 416, 428 to the fluid in the fluid conduit 430 thereby coolingthe heat sources 416, 420. The cooler 422 includes a duct 434 configuredto receive air from the atmosphere 18 and an air-stream heat exchanger436 located in the duct 434 to transfer heat between the fluid in thecooling loop 420 and the air passing through the duct 434. Thethermal-storage fuel system 424 is configured to transfer heat betweenthe fluid in the cooling loop 420 and fuel stored in the thermal-storagefuel system 424. The control system 426 is configured to control theflow of fluid through the fluid conduit 430.

The first heat source 416 is a high temperature heat source 416, whilethe second heat source 428 is a low temperature heat source 428. The lowtemperature heat source 428 may need to be kept cooler or at a lowertemperature than the high temperature heat source 416.

In the illustrative embodiment, the low temperature device 428 is abattery 428. In other embodiments, the low temperature device 428 may beanother low temperature device, such as solid state electronics, alight-emitting diode (“LED”), an analog circuit, a digital circuit, acomputer, a server, a server farm, a data center, a hoteling circuitsuch as vehicle electronics, a vehicle such as an aircraft, adirected-energy weapon, a laser, a plasma weapon, a microwave generator,a pulse-powered device, a satellite uplink, an electric motor orgenerator, an electric device, etc.

In the illustrative embodiment, the cooling loop 420 is structured toconduct the fluid heated from the first heat source 416 and the secondheat source 428 first through an engine-fuel heat exchanger 479. Thenthe fluid in the cooling loop 420 is conducted through the air-streamheat exchanger 436 after the engine-fuel heat exchanger 479. Then thefluid in the cooling loop 420 is conducted through a fuel-tank heatexchanger 448 after the air-stream heat exchanger 436.

In the illustrative embodiment, the cooling loop 420 further includes aplurality of expansion valves 440, 441 and a compressor 442. The firstexpansion valve 440 is arranged upstream of the low temperature device428. The second expansion valve 441 is arranged upstream of thethermal-storage fuel system 424. The compressor 442 is arrangeddownstream of the low temperature device 428.

In the illustrative embodiment, the cooling fluid in the cooling loop420 is a two-phase fluid. The expansion valves 440, 441, in conjunctionwith the compressor 442, are configured to reduce the pressure of thefluid in the cooling loop 420 between the expansion valves 440, 441 andthe compressor 442. By reducing the pressure of the fluid, thetemperature is lowered, which allows the fluid to effectively absorbheat.

In the illustrative embodiment, the cooling loop 420 is structured toconduct fluid through the second expansion valve 441 and through thefuel-tank heat exchanger 448 after the second expansion valve 441. Thenthe cooling loop 420 conducts the fluid through the compressor 442 afterthe fuel-tank heat exchanger 448, through the engine-fuel heat exchanger479 after the compressor 442, and through the air-stream heat exchanger436 after the engine-fuel heat exchanger 479.

The thermal-storage fuel system 424 includes fuel tanks 444, 446, thefuel-tank heat exchanger 448, and an engine-fuel unit 450 as shown inFIGS. 6 and 7. Both the first fuel tank 444 and second fuel tank 446 areconfigured to store fuel therein, while the fuel-tank heat exchanger 448is in thermal communication with the fluid conduit of the cooling loop420 to transfer heat between the fluid in the cooling loop 420 and fuelstored in the first fuel tank 444. The engine-fuel unit 450 isconfigured to receive fuel from the second fuel tank 446 and deliver thefuel to the engine 414. In the illustrative embodiment, the engine-fuelunit 450 is in thermal communication with the cooling loop 420 totransfer heat between the fluid in the cooling loop 420 and the fuel inthe engine-fuel unit 450.

The control system 426 includes a plurality of valves 454, 455, 456,457, 458, sensors 462, 463, 464, 466, 467, and a controller 468 as shownin FIGS. 6 and 7. The valves 454, 455, 456, 457, 458 are connected tothe fluid conduit 430 and configured to selectively control the flow offluid through the cooling loop 420. The sensors 462, 463, 464, 466, 467are configured to measure the temperature of the fuel in the fuel tanks444, 446 and the temperature of the fluid in the fluid conduit 430 ofthe cooling loop 420. The controller 468 is configured to operate theplurality of valves 454, 455, 456, 457, 458 to allow the fluid to flowthrough the fluid conduit 430 based at least in part on the informationmeasured by the sensors 462, 463, 464, 466, 467.

The controller 468 is configured to manage the cooling of the heat loads416 as well as the use of the thermal-storage fuel system 424. Thecontroller 468 is configured to direct the valves 454, 455, 456, 457,458 to vary the flow of the fluid in the cooling loop 420. Thecontroller 468 is also configured to control the fuel pump flow rate ofthe fuel pump 447 included in the thermal-storage fuel system 424through the fuel-tank heat exchanger 448. The controller 468 controlsthe valves 454, 455, 456, 457, 458 and the fuel pump 447 so as tomaintain the fluid in the cooling loop 420 at a predeterminedtemperature.

The plurality of valves 454, 455, 456, 457, 458 includes a first valve454, a second valve 456, a third valve 458, and modulating valves 455,457 as shown in FIGS. 6 and 7. The first valve 454 is connected to thefluid conduit 430 downstream of the air-stream heat exchanger 436 and isconfigured to selectively cause the flow of fluid to bypass thefuel-tank heat exchanger 448. The second valve 456 is connected to thefluid conduit 430 downstream of the first valve 454 and the expansionvalve 441 and is configured to selectively control the flow of fluidthrough the fuel-tank heat exchanger 448. The third valve 458 isconnected to the fluid conduit downstream of the fuel-tank heatexchanger 448 and is configured to selectively control the flow of fluidto the compressor 442 or back to the accumulator 438. The valves 455,457 are configured to vary the flow rate of the fluid in the coolingloop 420 through the air-stream heat exchanger 436.

The control system 426 is configured to selectively vary a flow of thefluid in the cooling loop 420 through the air-stream heat exchanger 436and the fuel-tank heat exchanger 448 to maintain a temperature of thehigh temperature heat source 416 below a predetermined heat loadtemperature.

In some embodiments, the controller 468 may be configured to determinean amount of heat transfer to the fuel in the first fuel tank 444 andoperate the plurality of valves 454, 455, 456, 457, 458 to selectivelyvary the flow of fluid in the cooling loop 420 through the air-streamheat exchanger 436 and the fuel-tank heat exchanger 448 in response tothe amount of heat transferred being different from a predetermined heatrejection schedule.

In some embodiments, the controller 468 is configured to determine atemperature of the fluid in the cooling loop 420 upstream of theaccumulator 438 included in the cooling loop 420. The controller 468 mayoperate the valves, 455, 457 to vary the flow of fluid in the coolingloop 420 through the air-stream heat exchanger 436 in response to thetemperature of the fluid in the cooling loop 420 upstream of theaccumulator 438 measured by the sensor 467 being different than a sensortarget temperature.

In the illustrative embodiment, the controller 468 is configured tooperate the plurality of valves 454, 455, 456, 457, 458 to allow thefluid to flow through the fuel-tank heat exchanger 448 based on inputsform the control system 426. The plurality of valves 454, 456, 458 allowor block fluid from flowing through the fuel-tank heat exchanger 448 inresponse to the controller 468 receiving different signals. Themodulating valves 455, 457 control the flow of fluid through theair-stream heat exchanger 436. The controller 468 may also be configuredto operate the expansion valves 440, 441 and the compressor 442 to allowthe fluid to flow through the expansion valves 440, 441 and thecompressor 442.

The plurality of valves 454, 456, 458 allow fluid to flow through thefuel-tank heat exchanger 448 if the controller 468 receives a firstsignal and/or a second signal. The first signal is indicative of atemperature of the fuel in the first fuel tank 444 being equal to orabove a predetermined maximum allowable tank temperature of the firstfuel tank 444, i.e. the maximum allowable temperature of the fuel in thefirst fuel tank 444 at that time of the mission. The second signal isindicative of a temperature of the cooling fluid directly upstream ofthe fuel-tank heat exchanger 448 being below the predetermined maximumallowable tank temperature, i.e. the maximum allowable temperature ofthe fuel in the first fuel tank 444 at that time of the mission.

The predetermined maximum allowable tank temperature, or the targettemperature for the first fuel tank 444, is the maximum allowabletemperature of the fuel in the first fuel tank 444 at that specific timeof the mission. In the illustrative embodiment, the controller 468 isconfigured to determine the changes of the predetermined maximumallowable tank temperature throughout the mission.

The maximum allowable fuel temperature for the engine 414 may alsochange throughout the mission. Therefore, the predetermined maximumallowable fuel temperature is the maximum allowable temperature of thefuel supplied to the engine 414 at that time of the mission. In theillustrative embodiment, the controller 468 is configured to determinethe changes of the predetermined maximum allowable fuel temperaturethroughout the mission.

In the illustrative embodiment, the controller 468 may be configureddetermine the amount of heat to add to or remove from the first fueltank 444 based on the predetermined maximum allowable fuel temperatureat different times over the course of the mission and control theplurality of valves 454, 456, 458 accordingly. Therefore, the pluralityof valves 454, 456, 458 may allow fluid to flow through the fuel-tankheat exchanger 448 based on the predetermined maximum allowable fueltemperature at different times over the course of the mission.

In the illustrative embodiment, the controller 468 is configured todetermine the changes of the predetermined maximum allowable tanktemperature throughout the mission. In some instances, the predeterminedmaximum allowable tank temperature may be greater than the predeterminedmaximum allowable fuel temperature.

In some embodiments, the plurality of valves 454, 456, 458 may allowfluid to flow through the fuel-tank heat exchanger 448 if the controller468 receives a third signal. The third signal may be indicative of theair temperature outside the aircraft. In other words, the third signalmay be indicative of the thermal energy system 12 being a predeterminedaltitude above ground where the air temperature is relatively cool. Inother embodiments, the air temperature at ground level may be coolenough to provide cooling to the fuel tanks 444, 446.

In the illustrative embodiments, the controller 468 is configured tooperate the plurality of valves 454, 456, 458 to allow the fluid to flowthrough the fuel-tank heat exchanger 448 in response to the controller468 receiving a fourth signal. The fourth signal is indicative of atake-off mode of the aircraft 10.

The engine-fuel unit 450 includes a mix valve 474, conduits 476, 478,and the engine-fuel heat exchanger 479 as shown in FIGS. 6 and 7. Thefirst conduit 476 is in fluid communication between the mix valve 474and the second fuel tank 446 to deliver fuel having a first temperatureto the mix valve 474. The second conduit 478 is in fluid communicationbetween the mix valve 474 and the second fuel tank 446. The engine-fuelheat exchanger 479 in thermal communication with the second conduit 478to cause the second conduit 478 to deliver fuel having a secondtemperature to the mix valve 474.

During use of the thermal energy system 412 in the aircraft 10, thefluid in the fluid conduit 430 is pumped from an accumulator 438included in the cooling loop 420 to the heat source 416 as indicted byarrow 482 and to the expansion valves 440, 441 as indicated by arrow 481in FIGS. 6 and 7. The pump 432 moves the fluid through the fluid conduit430 to transfer heat from the heat source 416 to the fluid to cool theheat source 416. The pump 432 also moves the fluid through the expansionvalve 440 as indicated by arrow 481 in FIGS. 6 and 7. The fluid isexpanded through the expansion valve 440 such that the fluid has atemperature lower than desired operating temperature of the battery 428,in order to remove heat from the battery 428.

Moving from the heat source 416, the flow of fluid in the fluid conduit430 moves through the engine-fuel heat exchanger 479 of the engine-fuelunit 450 as indicated by arrow 483 in FIGS. 6 and 7. The fluid in thefluid conduit 430 is in fluid communication with the engine-fuel heatexchanger 479 to transfer heat between the fluid to the fuel flowing andthe engine 414.

Simultaneously, as the fluid exits the expansion valve 440, the fluid inthe fluid conduit 430 moves across the low temperature heat loads 428 asindicated by arrow 494 as shown in FIGS. 6 and 7. As the fluid movesacross the low temperature heat loads 428, the low temperature heatloads 428 transfers heat to the fluid to cause the fluid to vaporize.The vapor than flows to the compressor 442 as indicated by arrow 496.The compressor 442 compresses the vapor in the fluid conduit 430 to thepressure that is in the conduit 430, and in the process increases thefluids temperature, in order for the fluid to be cooled by theengine-fuel heat exchanger 479 and/or the air-stream heat exchanger 436.

The fluid from the compressor 442 and the heat source 416 then flows tothe engine-fuel heat exchanger 479 as indicated by arrow 483 in FIGS. 6and 7. The fluid in the fluid conduit 430 may reject at least some heatto the fuel flowing through the engine-fuel heat exchanger 479 to coolthe fluid in the cooling loop 420. The mix valve 474 varies the flow offuel from the fluid conduits 476, 478 to adjust the temperature of thefuel provided to the engine 414.

As the fluid in the fluid conduit 430 exits the engine-fuel unit 450,some or all of the fluid flow may be passed to the air-stream heatexchanger 436 as indicated by arrow 484 as shown in FIGS. 6 and 7. Thefluid flows through the air-stream heat exchanger 436 to transferadditional heat from the fluid in the cooling loop 420 to the airpassing through the duct 434 of the cooler 422.

As the fluid in the fluid conduit 430 exits the air-stream heatexchanger 436 as indicated by arrow 486, the valve 454 controls the flowof fluid to the thermal-storage fuel system 424. The valve 454 isconfigured to selectively cause the flow of fluid to bypass thefuel-tank heat exchanger 448 based at least in part on the temperaturemeasured by the sensors 462, 463, 464, 466, 467 as discussed above andas discussed in the other embodiments. If the controller 468 detects thefirst signal and/or the second signal, the valve 454 allows fluid toflow through the fuel-tank heat exchanger 448 as indicated by arrow 487as shown in FIG. 6.

In the illustrative embodiment, as the valve 454 allows fluid in thefluid conduit 430 to flow to the fuel-tank heat exchanger 448, the valve454 partially or fully blocks flow from bypassing the fuel-tank heatexchanger 448 as suggested by the dotted line 492 in FIG. 6. As such,little to no fluid is flowing in the fluid conduit 430 in the region ofthe dotted line 492 in FIG. 6.

Conversely, the valve 454 blocks the flow of the fluid through thefuel-tank heat exchanger 448 based on the temperature measured by thesensors 462, 463, 464, 466, 467 as indicated by the dotted arrow 487shown in FIG. 7. If the valve 454 partially or fully blocks the fluid inthe fluid conduit 430 from flowing to the fuel-tank heat exchanger 448,the rest or all of the fluid flows to the accumulator 438 as indicatedby the arrow 492 in FIG. 7.

The valve 456 is connected to the fluid conduit 430 downstream of theexpansion valve 441 and the valve 454 as shown in FIGS. 6 and 7. Thevalve 456 is configured to selectively control the flow of fluid fromthe expansion valve 441 and the air-stream heat exchanger 436 based atleast in part on the temperature measured by the sensors 462, 464, 466as discussed above.

If the controller 468 detects the first and second signals, the valve456 is configured to allow all or some of the fluid in the cooling loop420 exiting the air-stream heat exchanger 436 to flow through thefuel-tank heat exchanger 448 as indicated by arrow 488 in FIG. 6.

Conversely, if the temperature of the fuel in the first fuel tank 444 isequal to or above the predetermined maximum allowable tank temperatureand the temperature of the air outside the aircraft 10 is relatively toohot, the valve 456 may fully, or partially block the fluid in thecooling loop 420 exiting the air-stream heat exchanger 436 through thefuel-tank heat exchanger 448. Similarly, if the temperature of the fuelin the first fuel tank 444 is equal to or above the predeterminedmaximum allowable fuel temperature and the temperature of the fluidexiting the air-stream heat exchanger 436 is above the predeterminedmaximum tank temperature, the valve 456 may fully, or partially blockfluid in the cooling loop 420 to flow through the fuel-tank heatexchanger 448.

In such instances, the expansion valve 441 may be allowed to providefluid to cool the fuel in the first fuel tank 444. The fluid from theexpansion valve 441 is able to further cool the fuel in the first fueltank 444 than what is possible with the air-stream heat exchanger 436.In some instances, the air flowing through the air-stream heat exchanger436 may be relatively high when the aircraft 10 and thus the thermalenergy system 12 is below the predetermined altitude.

In some embodiments, if the temperature of the fuel in the first fueltank 444 is equal to or above the predetermined maximum allowable tanktemperature and the air flowing through the air-stream heat exchanger436 is relatively high, the expansion valve 441 allows the fluid fromthe expansion valve 441 to flow through the fuel-tank heat exchanger 448as indicated by the arrow 488 in FIG. 7. Likewise, if the temperature ofthe fuel in the first fuel tank 444 is equal to or above thepredetermined maximum allowable fuel temperature and the temperature ofthe fluid exiting the air-stream heat exchanger 436 is above thepredetermined maximum tank temperature, the valve 456 allows fluid inthe cooling loop 420 from the expansion valve 441 to flow through thefuel-tank heat exchanger 448.

The fluid in the cooling loop 420 from the expansion valve 441 is at alower temperature than the temperature of the fluid at sensor 466, whichmay allow the fluid to remove heat from the fuel in the first fuel tank444 at a faster rate and/or to a lower temperature than if cooled withfluid coming from heat exchanger 436. The fluid in the cooling loop 420from the expansion valve 441 flowing through the fuel-tank heatexchanger 448 is vaporized and compressed. The third valve 458 controlsthe fluid in the cooling loop 420 as it exits the fuel-tank heatexchanger 448.

If the second valve 456 allowed the flow of fluid from the air-streamheat exchanger 436, the valve 458 directs the flow of fluid exiting thefuel-tank heat exchanger 448 (indicated by arrow 490) to return to theaccumulator 438 as indicated by arrow 493 in FIG. 6. As such, the valve458 fully blocks the flow of fluid to the compressor 442 as indicated bydotted arrow 497 in FIG. 6. Furthermore, a check valve 443 may be addeddownstream of the compressor 442, to prevent liquid from entering thecompressor 442 when the compressor 442 is not in operation.

Conversely, if the second valve 456 allows the flow of fluid from theexpansion valve 441, the valve 458 allows the flow of the fluid exitingthe fuel-tank heat exchanger 448 to flow to the compressor 442 asindicated by arrow 497 in FIG. 7. As such, the valve 458 fully blocksthe flow of fluid to the accumulator 438 as indicated by dotted line 493in FIG. 7.

A method of cooling heat loads on the aircraft 10 may include severalsteps. The method includes conducting the fluid through the cooling loop420 to transfer heat between the first heat source 416 and the fluid inthe cooling loop 420 and conducting the fluid through the cooling loop420 to transfer heat between the second heat source 428 and the fluid inthe cooling loop 420. The method also includes conducting the fluid inthe cooling loop 420 to the air-stream heat exchanger 436 to transferheat between the fluid in the cooling loop 420 and the air passingthrough the duct 434 and conducting the fluid in the cooling loop 420 tothe fuel-tank heat exchanger 448 to transfer heat between the fluid inthe cooling loop 420 and fuel stored in the first fuel tank 444.

In the illustrative embodiment, the method further includes selectivelyvarying a flow of the fluid in the cooling loop 420 through theair-stream heat exchanger 436 and a flow of the fluid through thefuel-tank heat exchanger 448. Varying the flow of the fluid through theair-stream heat exchanger 436 and the fuel-tank heat exchanger 448maintains a temperature of the first heat source 416 below thepredetermined heat load temperature.

In the illustrative embodiment, the method further comprises conductingthe fluid in the cooling loop 420 to the engine-fuel unit 450. The fluidin the cooling loop 420 is conducted to the engine-fuel unit 450 beforethe fluid is conducted to the air-stream heat exchanger 436 to transferheat between the fluid in the cooling loop 420 and the fuel in theengine-fuel unit 450.

In the illustrative embodiment, the method further includes varying aflow of fuel through the engine-fuel heat exchanger 479. The flow offuel is varied through the engine-fuel heat exchanger 479 to deliverfuel to the engine 414 at the engine-fuel unit predetermined thresholdtemperature.

Another embodiment of a thermal energy system 512 in accordance with thepresent disclosure is shown in FIG. 8. The thermal energy system 512 issubstantially similar to the thermal energy system 412 shown in FIGS. 6and 7 and described herein. Accordingly, similar reference numbers inthe 500 series indicate features that are common between the thermalenergy system 512 and the thermal energy system 412. The description ofthe thermal energy system 412 is incorporated by reference to apply tothe thermal energy system 512, except in instances when it conflictswith the specific description and the drawings of the thermal energysystem 512.

The thermal energy system 512 includes a cooling loop 520, a cooler 522,a thermal-storage fuel system 524, a control system 526, a first heatsource 516, and a second heat source 528 as shown in FIG. 8. The coolingloop 520 has a fluid conduit 530 and a pump 532 configured to move fluidthrough the fluid conduit 530 to transfer heat from the heat sources516, 528 to the fluid in the fluid conduit 530 thereby cooling the heatsources 516, 528. The cooler 522 includes a duct 534 configured toreceive air from the atmosphere 18 and an air-stream heat exchanger 536located in the duct 534 to transfer heat between the fluid in thecooling loop 520 and the air passing through the duct 534. Thethermal-storage fuel system 524 is configured to transfer heat betweenthe fluid in the cooling loop 520 and fuel stored in the thermal-storagefuel system 524. The control system 526 is configured to control theflow of fluid through the fluid conduit 530.

The control system 526 includes a plurality of valves 554, 555, 556,557, 558, 560, 561, sensors 562, 563, 564, 566, 567, and a controller568 as shown in FIG. 8. The valves 554, 555, 556, 557, 558, 560, 561 areconnected to the fluid conduit 530 and configured to selectively controlthe flow of fluid through the cooling loop 520. The sensors 562, 563,564, 566, 567 are configured to measure the temperature of the fuel inthe fuel tanks 544, 546 and the temperature of the fluid in the fluidconduit 530. The controller 568 is configured to operate the pluralityof valves 554, 555, 556, 557, 558, 560, 561 to allow the fluid to flowthrough the fluid conduit 530 based at least in part on the informationmeasured by the sensors 562, 563, 564, 566, 567.

The controller 568 is configured to manage the cooling of the heat loads516 as well as the use of the thermal-storage fuel system 524. Thecontroller 568 is configured to direct the valves 554, 555, 556, 557,558, 560, 561 to vary the flow of the fluid in the cooling loop 520. Thecontroller 568 is also configured to control the fuel pump flow rate ofthe fuel pump 547 included in the thermal-storage fuel system 524through the fuel-tank heat exchanger 548. The controller 568 controlsthe valves 554, 555, 556, 557, 560, 561 and the fuel pump 547 so as tomaintain the fluid in the cooling loop 520 at a predeterminedtemperature.

The plurality of valves 554, 555, 556, 557, 558, 560, 561 includes afirst valve 554, a second valve 556, a third valve 558, a fourth valve560, a fifth valve 561, and the modulating valves 555, 557 as shown inFIG. 8. The first valve 554 is connected to the fluid conduit 530downstream of the air-stream heat exchanger 536 so as to selectivelycause the flow of fluid to bypass the fuel-tank heat exchanger 548. Thesecond valve 556 is connected to the fluid conduit 530 downstream of thefirst valve 554 and an expansion valve 541 included in the cooling loop520 so as to selectively control the flow of fluid through the fuel-tankheat exchanger 548. The third valve 558 is connected to the fluidconduit 530 downstream of the fuel-tank heat exchanger 548 so as toselectively control the flow of fluid to the compressor 542 or back tothe accumulator 538. The fourth valve 560 is connected to the fluidconduit 530 upstream of a compressor 542 connected to the fluid conduit530 so as to selectively control the flow of fluid through thecompressor 542. The fifth valve 561 is connected to the fluid conduit530 upstream of an engine-fuel unit 550 included in the system 524 tovary the flow through the engine-fuel unit 550. The valves 555, 557 areconfigured to vary the flow rate of the fluid in the cooling loop 520through the air-stream heat exchanger 536.

The control system 526 is configured to selectively vary a flow of thefluid in the cooling loop 520 through the air-stream heat exchanger 536and the fuel-tank heat exchanger 548 to maintain a temperature of thehigh temperature heat source 516 below a predetermined heat loadtemperature.

In some embodiments, the controller 568 may be configured to determinean amount of heat transfer to the fuel in the first fuel tank 544 andoperate the plurality of valves 554, 555, 556, 557, 558 to selectivelyvary the flow of fluid in the cooling loop 520 through the air-streamheat exchanger 536 and the fuel-tank heat exchanger 548 in response tothe amount of heat transfer being different from a predetermined heatrejection schedule.

In some embodiments, the controller 568 is configured to determine atemperature of the fluid in the cooling loop 520 upstream of theaccumulator 538 included in the cooling loop 520. The controller 568 mayoperate the valves, 555, 557 to vary the flow of fluid in the coolingloop 520 through the air-stream heat exchanger 536 in response to thetemperature of the fluid in the cooling loop 520 upstream of theaccumulator 538 measured by the sensor 567 being different than a sensortarget temperature.

In the illustrative embodiment, the second expansion valve 541 isarranged upstream of the thermal-storage fuel system 524. The compressor542 is arranged downstream of the low temperature heat loads 528. In theillustrative embodiment, the expansion valves 540, 541 may act ascontrol valves to control the flow of the fluid across the lowtemperature heat loads 528. In other embodiments, the expansion valves540, 541, in conjunction with the compressor 542, may be configured toreduce the pressure of the fluid in the cooling loop 520 between theexpansion valves 540, 541 and the compressor 542.

The thermal-storage fuel system 524 further includes the engine-fuelunit 550 as shown in FIG. 8. Both the first fuel tank 544 and secondfuel tank 546 are configured to store fuel therein. The engine-fuel unit550 is configured to receive fuel from the second fuel tank 546 anddeliver the fuel to the engine 514. In the illustrative embodiment, theengine-fuel unit 550 is in thermal communication with the cooling loop520 to transfer heat between the fluid in the cooling loop 520 and thefuel in the engine-fuel unit 550.

The engine-fuel unit 550 includes a mix valve 574, conduits 576, 578,and an engine-fuel heat exchanger 579 as shown in FIG. 8. The firstconduit 576 is in fluid communication between the mix valve 574 and thesecond fuel tank 546 to deliver fuel having a first temperature to themix valve 574. The second conduit 578 is in fluid communication betweenthe mix valve 574 and the second fuel tank 546. The engine-fuel heatexchanger 579 in thermal communication with the second conduit 578 tocause the second conduit 578 to deliver fuel having a second temperatureto the mix valve 574.

In the illustrative embodiment, the controller 568 is configured tooperate the plurality of valves 554, 555, 556, 557, 558, 560, 561 tocontrol the fluid to flow through the fuel-tank heat exchanger 548, theengine-fuel unit 550, and the compressor 542 based on inputs form thecontrol system 526. Similar to the embodiments of FIGS. 6 and 7, theplurality of valves 554, 556, 558 allow fluid to flow through thefuel-tank heat exchanger 548 if the controller 568 receives theappropriate signals. In the illustrative embodiment, the controller 568is also configured operate the fourth valve 560 to control the fluidthrough the compressor 542 based on inputs from the control system 526.

During use of the thermal energy system 512 in the aircraft 10, thefluid in the fluid conduit 530 is pumped from an accumulator 538included in the cooling loop 520 to the heat source 516 as indicted byarrow 582 and to the expansion valves 540, 541 as indicated by arrow 581in FIG. 8. The pump 532 moves the fluid through the fluid conduit 530 totransfer heat from the heat source 516 to the fluid to cool the heatsource 516. The pump 532 also moves the fluid through the expansionvalve 540 to expand the fluid such that the fluid has a relatively lowtemperature compared the low temperature heat loads 528.

As the fluid exits the expansion valve 540, the fluid in the fluidconduit 530 moves across the low temperature heat loads 528 as indicatedby arrow 594 as shown in FIG. 8. As the fluid moves across the lowtemperature heat loads 528, the low temperature heat loads 528 transferheat to the fluid to cause the fluid to be a vapor. The vapor than flowsto the compressor 542 as indicated by arrow 596. The compressor 542compresses the vapor in the fluid conduit 530 to be cooled by thethermal-storage fuel system 524 or the engine-fuel heat exchanger 579and/or the air-stream heat exchanger 536 in some embodiments.

However, on colder days, thermal lift may not be needed, as the airtemperature may be low enough to sufficiently cool the low temperatureheat loads 528. In this case, the accumulator 538 may be operated at alower temperature without the need of the compressor 542 to lower thepressure and temperature in the low temperature heat loads 528. As such,the expansion valve 540 may operate as flow control valves and the fluidpassing through the low temperature heat loads 528 may be vaporized. Theresulting vaporized or partially vaporized fluid may then bypass thecompressor 542 as indicated by the dotted arrow 598 as shown in FIG. 8,which indicates no fluid flow through the compressor 542.

In some embodiments, the valve 560 may be eliminated and the fluid maybe passed through the compressor 542 that is not running. However, thevalve 560 may be included in the event that stopping the compressor 542would not permit the flow of fluid through the compressor 542.Furthermore, a check valve 543 may be added downstream of the compressor542, to prevent liquid from entering the compressor 542 when thecompressor 542 is not in operation.

Instead, the controller 568 is configured to direct the valve 560 todirect the vaporized or partially vaporized fluid back to theaccumulator 538. The valve 560 therefore blocks the flow of fluid to thecompressor 542 as indicated by the dotted arrow 598.

In the illustrative embodiment, the cooling loop 520 further includes anorifice 531 as shown in FIG. 8. The orifice 531 is connected to thefluid conduit 530 upstream of the heat source 516 to increase the pumpexit pressure of the pump 532. Increasing the pump exit pressure forcesthe flow through the low temperature heat loads 528. Depending on thepressure drops through the various components 516, 548, 528, etc., theorifice 531 may be removed. In some embodiments, the controller 568 maybe configured to control the orifice 531.

The present disclosure relates to an aircraft 10 with a thermal energysystem 12 to cool low temperature heat loads of the electricalcomponents or other engine or aircraft oil/hydraulic heat loads. Hybridelectric thermal energy systems may rely on ram-air heat exchangers forrejecting the waste heat produced by power electronic devices orelectric machines such as generators and motors.

In the illustrative embodiments, the gas turbine engine 14 is mounted tothe outside of the aircraft 10. In other embodiments, the gas turbineengine 14, 214, 314, 414, 514 may be inside of the aircraft 10. Theengine 14, 214, 314, 414, 514 may be in the fuselage where electricpower is produced. This electric power may then be transferred to anelectric fan. The electric fan may be in other parts of the aircraft 10,such as the tail, or there may be may electric fans distributed on thewings of the aircraft 10.

The thermal energy system 12 may use single-phase liquid coolants ortwo-phase coolants. For two-phase coolants, the cooling loop 20, 220,320, 420, 520 may represent a two-phase pump loop (TPPL). The use of theram air may be driven by the low temperature requirements of the devicesbeing cooled. This low temperature difference between the componentbeing cooled and the ram air temperature causes several challenges. Inother embodiments, a fan stream duct may be used.

The controller 68, 268, 468, 568 may be configured to direct the valves54, 254, 454, 456, 458, 554, 556, 558 to selectively control the flow offluid in the cooling loop 20, 220, 420, 520 through the fuel-tank heatexchanger 48, 248, 448, 548 to maximize fuel savings of the aircraft 10based on the type of day (i.e. hot day or cold day). The controller 68,268, 468, 568 may direct the valves 54, 254, 454, 456, 458, 554, 556,558 based on the temperature along the flight path, while keeping thehigh temperature heat sources 16, 216, 316, 416, 516 and/or fuel below apredetermined threshold.

First, when the aircraft 10 is sitting on the ground or taxiing, thereis little to no ram pressure to force air through the air-stream heatexchanger 36 to cool the coolant. As such, a typical aircraft system mayinclude a blower to force air through the heat rejection heat exchanger.The blower and additional ducting adds weight and volume to the thermalmanagement system. The system of the present disclosure does not includea blower.

Second, the highest heat loads typically occur at take-off, when the ramair is quite close in temperature to the devices to be cooled. Thereforevery large mass flow rates of air may be needed to carry the heat away.Furthermore, at start of take-off, the ram pressure is relatively low.The large airflow rates and low ram pressure rise may cause typical heatrejection heat exchangers and ram air ducts to be relatively large insize. The ducts and heat exchangers of the present disclosure may besmall in size to reduce weight.

Conversely, at cruise altitude, the ram air temperature will havesignificantly decreased and the ram pressure difference on the heatexchanger will have increased. As a result, a much smaller heatexchanger could theoretically be used on typical aircraft. However,because the heat rejection heat exchanger may have been sized for hotday take-off, the typical aircraft may experience a large drag penalty.

To eliminate the blower and reduce the size of the duct and heatrejection heat exchanger, the thermal energy system 12, 212, 312, 412,512, 612 of the present disclosure includes a thermal-storage fuelsystem 24, 224, 324, 424, 524, 624 that increases the efficiency of theaircraft 10.

The thermal-storage fuel system 24, 224, 324, 424, 524 enables a smallerram air cooler 22, 222, 322, 422, 522 and eliminates the blower. Thethermal-storage fuel system 24, 224, 324, 424, 524 uses the availableon-board fuel as a heat sink for all or a portion of the electricalloads produced early in the mission while at low altitude (e.g. idle,taxi, take-off).

Then, at altitude, the heat stored in the thermal-storage fuel system24, 224, 324, 424, 524 may be rejected to the much colder ambient air.As a result, the air-stream heat exchanger 36, 236, 336, 436, 536 andthe duct 34, 234, 334, 434, 534 may be sized for a condition that is notas challenging as the worst case condition of rejecting high heat loadsto hot air temperatures with little ram pressure, resulting in smaller,lighter, and more efficient thermal system 12, 212, 312, 412, 512.

The heat transferred to the fluid in the cooling loop 20, 220, 320, 420,520 is removed from the fluid using several different methods. In someembodiments, the engine-fuel unit 250, 350, 450, 550 rejects heat to thefuel that is being passed to the engine 214, 314, 414, 514 to be burned.In some embodiments, the ram-air heat rejection heat exchanger 36, 236,336, 436, 536 rejects heat to the air flowing through the duct 34, 234,334, 434, 534. In some embodiments, the thermal-storage fuel system 24,224, 324, 424, 524 stores the heat from the fluid in the fuel in thefirst fuel tank 44, 244, 344, 444, 544. Any combination of these heatmanagement features may be used. Each of these heat rejection methodshelps remove heat from the heat source 16, 216, 316, 416, 516 out of thesystem 12, 212, 312, 412, 512 at different points in the mission orduring different flight conditions.

In some conventional thermal energy systems, the fuel is transferredfrom the tank, heated by the heat loads of the system, possibly cooled,and then returned to the tank. Moving the fuel around the system may behazardous and cause safety issues.

In this disclosure, the thermal-storage fuel system 24, 224, 324, 424,524 keeps the fuel in close proximity to the respective fuel tank 44,244, 344, 444, 544 and eliminates the running of fuel around theaircraft 10, which reduces the risk of fire. In the illustrativeembodiments of FIGS. 2-5, the system 12, 312 uses a single-phase coolant(e.g. water based coolant, oil, etc.). However, in the embodiments ofFIGS. 2-4, the system 12 may also use a two-phase coolant.

In the illustrative embodiments of FIGS. 6-8, the system 212, 412, 512uses a two-phase coolant. In the case of two-phase coolant, the pumpingpower and pump size may also be decreased relative to the pumping ofsingle phase coolant.

In the illustrative embodiments, the thermal-storage fuel system 24,224, 324, 424, 524 includes a dedicated thermal management system (TMS)tank 44, 244, 344, 444, 544 configured to enable a portion of the fuelto be taken to a temperature higher than what the engine 14, 214, 314,414, 514 may safely receive. In this way, the fuel heat storage capacityis increased. The fuel may later be cooled to a lower temperature to beable to be safely supplied to the engine 14, 214, 314, 414, 514 later inthe mission.

In the illustrative embodiments, the fuel-tank heat exchanger 48, 248,348, 448, 548 is shown to be located in the TMS tank 44, 244, 344, 444,544. In other embodiments, the fuel-tank heat exchanger 48, 248, 348,448, 548 may be located adjacent to or outside of the TMS tank 44, 244,344, 444, 544. The fuel-tank heat exchanger 48, 248, 348, 448, 548 maycontact the outside of the TMS fuel tank 44, 244, 344, 444, 544 in someembodiments.

In other embodiments, the fuel-tank heat exchanger 48, 248, 348, 448,548 is located adjacent to or in close proximity to the thermal-storagefuel system 24, 224, 324, 424, 524. In other embodiments, the fuel-tankheat exchanger 48, 248, 348, 448, 548 may be located in another suitablelocation and fluidly connected to the fluid conduit 30, 230, 330, 430,530 of the cooling loop 20, 220, 320, 420, 520.

The thermal energy system 12, 212, 312, 412, 512 extends a simpletwo-phase pump loop architecture to move and dissipate heat to variousstations along the TPPL. The thermal-storage fuel system 24, 224, 324,424, 524 stores the heat load on board and the air-stream heat exchanger36, 236, 336, 436, 536 dissipates the heat to air when it can be donemore efficiently. This reduces the size and drag of the condenser on theaircraft 10.

In the illustrative embodiments of FIGS. 6-8, the thermal energy system412, 512 further includes a compressor 442, 542 to enable cooling thelow temperature heat loads 428, 528 to a lower temperature. Thecompressor 442, 542 may provide the capability of cooling the lowtemperature heat loads on a hot day. The compressor 442, 542 may alsoallow the other heat loads to operate at higher temperatures, thusmaking greater use of the thermal-storage fuel system 424, 524. In otherembodiments, the low temperature heat loads 428, 528 may be another lowtemperature device, such as avionics, directed energy weapons, etc.

At the engine-fuel unit 250, 350, 450, 550, the coolant rejects heat tothe fuel provided to the engine 214, 314, 414, 514 to be burned. Inorder to maximize the use of the fuel heat sink, the fuel entering theengine 214, 314, 414, 514 is increased in temperature to its maximumallowable value or maximum allowable fuel temperature.

To ensure the fuel provided to the engine 214, 314, 414, 514 is at themaximum allowable fuel temperature, the fuel stream is split prior tothe engine-fuel heat exchanger 279, 479, 579 and only a portion of thefuel passes through the engine-fuel heat exchanger 279, 479, 579. A mixvalve 274, 474, 574 configured to sense the temperature of the fuelexiting the mix valve 274, 474, 574 is used to manage the temperature ofthe fuel entering the engine 214, 414, 514.

The mix valve 274, 474, 574 varies the amount of flow that passesthrough the engine-fuel heat exchanger 279, 479, 579. If the coolanttemperature is just hot enough to heat the fuel to the maximum allowablefuel temperature, the mix valve 274, 474, 574 will pass all of the fuelthrough the engine-fuel heat exchanger 279, 479, 579.

If the fuel coming from the second fuel tank 246, 446, 546 is already atthe maximum allowable fuel temperature, then no fuel will pass throughthe engine-fuel heat exchanger 279, 479, 579. In some embodiments, thesplit may be somewhere between these two extremes.

The thermal-storage fuel system 24, 224, 324, 424, 524 includes theaircraft fuel tanks 44, 46, 244, 246, 344, 346, 444, 446, 544, 546 wheresome of the heat loads in the coolant are rejected. The fuel tanks 44,46, 244, 246, 344, 346, 444, 446, 544, 546 are divided into at least twofunctional tanks (each functional tank can be made up of multipletanks). The second fuel tank 46, 246, 346, 446, 546 may be very similarto fuel tanks typically used on aircraft 10.

However, the first fuel tank 44, 244, 344, 444, 544 may be a dedicatedfuel tank(s) configured to isolate a portion of the aircraft fuel. Indoing so, this fuel may be heated to predetermined threshold temperatureor maximum allowable tank temperature. This temperature may be higherthan a maximum allowable tank temperature of the second fuel tank 46,246, 346, 446, 546 and the maximum allowable fuel temperature. In someembodiments, the maximum temperature in the first fuel tank 44, 244,344, 444, 544 may be limited to the highest temperature of the coolantin the cooling loop 20, 220, 320, 420, 520.

However, if the coolant maximum temperature is too high, an upper limitmay be put on the maximum allowable tank temperature of the first fueltank 44, 244, 344, 444, 544. Later in the mission, the fuel in the firstfuel tank 44, 244, 344, 444, 544 may be cooled to or below maximumallowable tank temperature of the first fuel tank 44, 244, 344, 444,544, so that the engine 14, 214, 314, 414, 514, may safely receive thefuel. In some embodiments, a fuel pump may be used to pump fuel throughthe fuel-tank heat exchanger 48, 248, 348, 448, 548.

In some embodiments, the heat loads may be low enough that the firstfuel tank 44, 244, 344, 444, 544 may not need to be heated higher thanmaximum allowable tank temperature of the second fuel tank 46, 246, 346,446, 546. In this case, the dedicated first fuel tank 44, 244, 344, 444,544 may not be needed.

In other embodiments, the fuel may be mixed between the second fuel tank46, 246, 346, 446, 546 and the first fuel tank 44, 244, 344, 444, 544until the second fuel tank 46, 246, 346, 446, 546 reaches maximumallowable tank temperature of the second fuel tank 46, 246, 346, 446,546. In doing so the time before the first fuel tank 44, 244, 344, 444,544 reaches the maximum allowable tank temperature of the first fueltank 44, 244, 344, 444, 544, is extended. This allows the aircraft 10 toreach a higher altitude (and hence colder air) before all cooling issupplied by the air-stream heat exchanger 36, 236, 336, 436, 536.

In the illustrative embodiment, the fuel-tank heat exchanger 48, 248,348, 448, 548 is located in the first fuel tank 44, 244, 344, 444, 544.However, in other embodiments, the fuel-tank heat exchanger 48, 248,348, 448, 548 may be located in another suitable location to heat thefuel in the first fuel tank 44, 244, 344, 444, 544.

The ram air cooler 22 includes a variable NACA inlet 33. The variableNACA inlet 33 will reduce the amount of drag created from the duct 34.

Furthermore, on very cold days when the fuel is very cold, it may beadvantageous to minimize heat rejection via the ram air cooler 22, 222,322, 422, 522 to minimize drag and to dump the heat to the fuel. In thissituation, the variable inlet 33 may be closed off, forcing theremaining coolant heat to be rejected to the first fuel tank 44, 244,344, 444, 544. Therefore, some of the waste heat may be retained in theengine cycle, improving system efficiency.

Unlike other ram air coolers, the blower and blower duct/door areremoved from the ram air cooler 22, 222, 322, 422, 522 of the presentdisclosure. This first fuel tank 44, 244, 344, 444, 544 is instead usedas thermal energy storage when at ground idle or when taxiing.

Because the air stream may not be needed until later in the mission,when at a higher altitude, the fan stream in an electric propulsor maybe sufficiently cool to use as the air heat sink. If this is the case, adedicated ram air duct may not be needed.

To help with understanding the operation of this system 12, 212, 312,412, 512, the different modes of operation through a mission will bedescribed. The mission to be described is for a very hot day.

At ground idle, taxi, take-off, or start of climb, very little heat maybe rejected through the air-stream heat exchanger 36, 236, 336, 436,536. As much heat as possible may be transferred to the fuel through theengine-fuel heat exchanger 279, 479, 579, most of the heat may berejected through the fuel-tank heat exchanger 48, 248, 348, 448, 548 andstored in the fuel in the first fuel tank 44, 244, 344, 444, 544.

As the aircraft 10 climbs and increases in speed, the air in theatmosphere 18 may become colder and the pressure difference across theair-stream heat exchanger 36, 236, 336, 436, 536 will increase.Therefore, even more heat form the coolant may be rejected through theair-stream heat exchanger 36, 236, 336, 436, 536 and less heat will berejected to the first fuel tank 44, 244, 344, 444, 544. At some point,all the heat may be rejected from the engine-fuel unit 250, 350, 450,550 and the ram air cooler 22, 222, 322, 422, 522.

At the end of the climb, as the aircraft beings to cruise, the coolantexiting the ram air cooler 22, 222, 322, 422, 522 may have lost moreheat than is being added to the coolant from the heat source 16, 216,316, 416, 516. At this point, the coolant from the air-stream heatexchanger 36, 236, 336, 436, 536 flowing to the fuel-tank heat exchanger48, 248, 348, 448, 548 may begin to absorb heat from the fuel in thefirst fuel tank 44, 244, 344, 444, 544.

During the cruise phase of the mission, the first fuel tank 44, 244,344, 444, 544 may be chilled below maximum allowable fuel temperature sothat the fuel may be burned by the engine 14, 214, 314, 414, 514. It mayalso be useful to significantly lower the fuel temperature to a very lowtemperature. In other words, the first fuel tank 44, 244, 344, 444, 544may be deeply cooled before landing.

As the aircraft 10 descends to lower altitudes, the air through the duct34, 234, 334, 434, 534 may begin to heat up again. During descent, heatsource 16, 216, 316, 416, 516 may be significantly less than duringclimb. However, a mission optimized ram air duct 34, 234, 334, 434, 534may not be able to provide all of the needed cooling during the fullfinal portion of the mission.

As a result, the first fuel tank 44, 244, 344, 444, 544 may be used asheat storage once again. At this point, there is only a portion of theearlier fuel remaining. This remaining fuel may be made up of fuel yetto be burned and fuel reserves. However, if the fuel had been previouslydeeply cooled to a low temperature, the limited fuel mass may stillprovide significant thermal storage because it is so cold relative tomaximum allowable fuel temperature.

A variation on the mission profile may be for cold days. On cold days,the fuel may provide a very large thermal heat sink. In this case, onemay close off the ram air cooler 22, 222, 322, 422, 522 and reject moreof the heat into the very cold fuel in the first fuel tank 44, 244, 344,444, 544. Furthermore, because the second fuel tank 46, 246, 346, 446,546 is so cold, rejecting heat into the second fuel tank 46, 246, 346,446, 546 may be advantageous. In this way, electrical energy losses maybe returned to the fuel heat sink and minimize energy that may be dumpedoverboard. Furthermore, by avoiding use of the ram air cooler 22, 222,322, 422, 522, ram drag affects may be minimized.

In the illustrative embodiment of FIGS. 6 and 7, the system 412 includesbatteries 428, which operate at relatively low temperatures. Theillustrative embodiment, may add thermal lift capability to providecooling capability at lower temperatures for the batteries 428.

In such illustrative embodiments, the coolant may be operated in twophases (i.e. liquid and vapor). The coolant may be a refrigerant that ischosen for optimal efficiency in the temperature operating range of thecomponents being cooled and the heat sink that the heat is rejected to.

The thermal energy system 412 includes expansion valves 440, 441 and acompressor 442. The compressor 442 lowers the pressure downstream of theexpansion valves 440, 441 indicated by arrows 496, 485. As the liquidrefrigerant is expanded across the expansion valve 440, the refrigeranttemperature is lowered and is able to keep the batteries 428 at a lowertemperature than the other heat loads. As the refrigerant absorbs heatin the batteries 428, the coolant is vaporized and the compressor 442then compresses the refrigerant to return it to the coolant to becooled.

On cold days, thermal lift may not be needed, because the airtemperature may be low enough to sufficiently cool the batteries 428,528 using a two-phase pump loop, without thermal lift. In that case, theexpansion valve 440 will have a lower pressure drop and will be used tocontrol the flow rate to the batteries 428, 528. As the coolant ispassed through the low temperature heat loads 428, the coolant may bepartially or fully vaporized.

In the illustrative embodiment of FIG. 8, the resulting two-phase flowmay then bypass the compressor 542 and be returned to the accumulator538. This approach would likely use an orifice 531 just before the heatsource 516 to increase the pump exit pressure in order to force flowthrough the batteries 528. The fluid flowing through the batteries 528is then directed back to the accumulator 538 by the valve 560. In thismode of operation, the full system may be behaving as a TPPL.

Alternatively, the valve 560 may allow the fluid in line 596 to flowthrough line 598 to the compressor. The fluid may then flow throughcompressor 542 to eventually pass through line 583 to the engine-fuelheat exchanger 579. The compressor 542 may have a bypass line to allowfluid to flow in this way if the compressor 542 is of a positivedisplacement design.

In the illustrative embodiments of FIGS. 6 and 7, the valves 454, 456,458 controls the flow of the fluid to and from the fuel-tank heatexchanger 448. In some embodiments, the low temperature expandedrefrigerant may be supplied to the fuel-tank heat exchanger 448 as showin FIG. 7. This may enable dropping the temperature of the fuel in thefirst fuel tank 444 to a temperature lower than the air temperature inthe cooler 422. This may be useful on very hot day operations to drivethe fuel temperature to a very low temperature in preparation for usingit as a heat sink at the end of the mission.

The control system 26, 226, 426, 526 is configured to control the flowof fluid in the cooling loop 20, 220, 420, 520 to improve the overallperformance of the aircraft 10. To do so, the first step may be tomaintain acceptable operating temperatures on the devices being cooled.The second may be to minimize fuel burn over the entire mission.

In doing this, the controller 68, 268, 468, 568 may chose when or how toreject the heat load 16, 216, 416, 516, 616 from the cooling loop 20,220, 420, 520. Hence, the controller 68, 268, 468, 568 may be configuredto put as much heat into the fuel tanks 44, 46, 244, 246, 344, 346, 444,446, 544, 546, without ever encountering a situation where the devicesare hotter than some upper predetermined threshold limit.

On a hot day, on a short duration flight, the use of the first fuel tank44, 244, 344, 444, 544 may be minimized so that during descent and/ortaxiing, the first tank 44, 244, 344, 444, 544 is cold enough to providecooling.

However, on a longer flight, perhaps more heat may be added to the firsttank 44, 244, 344, 444, 544. During the long flight, the first tank 44,244, 344, 444, 544 may start being cooled with a higher initialtemperature because there is sufficient time to cool the tank 44, 244,344, 444, 544 back down to the temperature required during descent andtaxiing.

In the next extreme, on a cold day flight, the fuel tanks 44, 46, 244,246, 344, 346, 444, 446, 544, 546 may be heated to minimize drag fromthe ram duct 34, 234, 334, 434, 534 and to minimize waste heat frombeing dumped overboard of the aircraft 10. As such, the controller 68,268, 468, 568 may be configured to look at the mission profile andweather along the flight path to minimize fuel burn while stillmaintaining acceptable temperatures on the components.

In the illustrative embodiments, the sensors 62, 63, 64, 66, 67, 262,263, 264, 266, 267, 462, 463, 464, 466, 467, 562, 563, 564, 566, 567 areshown in respective locations to measure the temperature of the fuel orfluid in the system 12, 212, 312, 412, 512. In other embodiments, thesensors 62, 63, 64, 66, 67, 262, 263, 264, 266, 267, 462, 463, 464, 466,467, 562, 563, 564, 566, 567 may be located along other points along thefluid conduit 30, 230, 430, 530, 630 that are suitable to measure thetemperature of the fluid in the cooling loop 20, 220, 420, 520. In otherembodiments, the system 12, 212, 412, 512 may include more sensors.

While the disclosure has been illustrated and described in detail in theforegoing drawings and description, the same is to be considered asexemplary and not restrictive in character, it being understood thatonly illustrative embodiments thereof have been shown and described andthat all changes and modifications that come within the spirit of thedisclosure are desired to be protected.

What is claimed is:
 1. A thermal energy system for use with an aircraft,the thermal energy system comprising a first heat source, a second heatsource configured to operate at a lower temperature than the first heatsource during use of the thermal energy system, a cooling loop having afluid conduit, a pump configured to move fluid through the fluid conduitto transfer heat from the first heat source and the second heat sourceto the fluid to cool the first heat source and the second heat source, acompressor located downstream of the second heat source, and a firstexpansion valve located upstream of the second heat source, a coolerthat includes a duct configured to conduct air through the duct and anair-stream heat exchanger located in the duct and in thermalcommunication with the fluid in the cooling loop to transfer heatbetween the cooling loop and the air conducted through the duct, and athermal-storage fuel system that includes a first fuel tank configuredto store fuel therein and a fuel-tank heat exchanger in thermalcommunication with the fluid and configured to transfer heat between thefluid in the cooling loop and fuel stored in the first fuel tank.
 2. Thethermal energy system of claim 1, wherein the thermal-storage fuelsystem further includes a second fuel tank in fluid communication withthe first fuel tank, the second fuel tank configured to store fuel at atemperature that is different from a temperature of the fuel in thefirst fuel tank.
 3. The thermal energy system of claim 2, wherein thethermal-storage fuel system further includes an engine-fuel unitconfigured to receive fuel from the second fuel tank and deliver thefuel to an engine and the engine-fuel unit is in thermal communicationwith the cooling loop to transfer heat between the fluid in the coolingloop and the fuel in the engine-fuel unit.
 4. The thermal energy systemof claim 3, wherein the engine-fuel unit includes a valve system influid communication with the second fuel tank and an engine-fuel heatexchanger in thermal communication with the valve system and the coolingloop to transfer heat between the fluid in the cooling loop and the fuelfrom the second fuel tank, and wherein the valve system is configured tovary a flow of fuel through the engine-fuel heat exchanger to deliverfuel to the engine at an engine-fuel unit predetermined thresholdtemperature.
 5. The thermal energy system of claim 3, wherein thecooling loop is structured to conduct the fluid heated from the firstheat source and the second heat source first through the engine-fuelheat exchanger, through the air-stream heat exchanger after theengine-fuel heat exchanger, and through the fuel-tank heat exchangerafter the air-stream heat exchanger, and wherein the control system isconfigured to measure the heat transfer between the fluid in the coolingloop and the fuel in the first fuel tank, compare the heat transfermeasured to a predetermined heat rejection schedule, and selectivelyvary the flow of fluid in the cooling loop through the fuel-tank heatexchanger in response to the heat transfer measured being different fromthe predetermined heat rejection schedule.
 6. The thermal energy systemof claim 1, wherein the cooling loop is structured to conduct fluidthrough the first expansion valve, through the second heat source afterthe first expansion valve, and through the compressor after the secondheat source to lower a temperature of the fluid in the cooling loop sothat heat produced by the second heat source is removed from the coolingloop.
 7. The thermal energy system of claim 6, wherein the cooling loopfurther includes a second expansion valve located upstream of thefuel-tank heat exchanger.
 8. The thermal energy system of claim 7,wherein the cooling loop is structured to conduct fluid through thesecond expansion valve, through the fuel-tank heat exchanger after thesecond expansion valve, and through the compressor after the fuel-tankheat exchanger.
 9. The thermal energy system of claim 1, wherein thecooling loop includes a plurality of valves connected to the fluidconduit and configured to selectively cause at least a portion of a flowof fluid in the cooling loop to bypass the fuel-tank heat exchanger. 10.The thermal energy system of claim 9, wherein the cooling loop includesmodulating valves connected to the fluid conduit and configured to varythe flow of fluid in the cooling loop through the air-stream heatexchanger.
 11. The thermal energy system of claim 1, wherein the secondheat source is at least one of a battery, an avionic system, and adirected energy weapon.
 12. The thermal energy system of claim 1,further comprising a control system configured to selectively vary aflow of the fluid in the cooling loop through the air-stream heatexchanger and the fuel-tank heat exchanger to maintain a temperature ofthe first heat source below a predetermined heat load temperature. 13.The thermal energy system of claim 12, wherein the control systemincludes a plurality of valves connected to the fluid conduit and acontroller connected to the plurality of valves, the controllerconfigured to determine an amount of heat transfer between the fluid andthe fuel in the first fuel tank and operate the plurality of valves toselectively vary the flow of fluid in the cooling loop through thefuel-tank heat exchanger in response to the amount of heat transferbeing different from a predetermined heat rejection schedule.
 14. Thethermal energy system of claim 13, wherein the controller is configuredto operate the plurality of valves to at least partially block the fluidfrom flowing through the fuel-tank heat exchanger in response to thecontroller receiving a signal indicative of the temperature of the fuelin the first fuel tank being equal to or above a first fuel tankpredetermined threshold temperature.
 15. The thermal energy system ofclaim 13, wherein the controller is configured to operate the pluralityof valves to allow the fluid to flow through the fuel-tank heatexchanger in response to the controller receiving i) a signal indicativeof a temperature of the fuel in the first fuel tank being equal to orabove a first fuel tank predetermined threshold temperature and ii) asignal indicative of a temperature of the cooling fluid being less thana predetermined maximum allowable tank temperature, and wherein thecontrol system is configured to vary the predetermined maximum allowabletank temperature throughout a flight cycle of the aircraft.
 16. Thethermal energy system of claim 1, further comprising a control systemthat includes a plurality of valves connected to the fluid conduit and acontroller connected to the plurality of valves, the controllerconfigured to determine a temperature of the fluid in the cooling loopupstream of an accumulator included in the cooling loop and operate theplurality of valves to selectively vary the flow of fluid in the coolingloop through the air-stream heat exchanger and through the fuel-tankheat exchanger in response to the temperature of the fluid in thecooling loop upstream of the accumulator being different from a sensortarget temperature.
 17. The thermal energy system of claim 1, whereinthe cooler is a ram air cooler and the duct is configured to receive airfrom atmosphere around the aircraft during forward movement of theaircraft relative to ground and is free of any air mover.
 18. A methodcomprising providing a thermal energy system for use with an aircraftincluding a cooling loop, a cooler, and a thermal-storage fuel system,the cooling loop having a fluid conduit, a pump configured to move fluidthrough the fluid conduit, and a compressor, the cooler including a ductconfigured to conduct air through the duct and an air-stream heatexchanger located in the duct and in thermal communication with thefluid in the cooling loop to transfer heat between the cooling loop andthe air conducted through the duct, and the thermal-storage fuel systemincluding a first fuel tank configured to store fuel therein and afuel-tank heat exchanger in thermal communication with the fluid in thecooling loop, conducting the fluid through the cooling loop to transferheat between a first heat source and the fluid in the cooling loop,conducting the fluid through the cooling loop to transfer heat between asecond heat source and the fluid in the cooling loop, conducting thefluid in the cooling loop to the air-stream heat exchanger to transferheat between the fluid in the cooling loop and the air passing throughthe duct, and conducting the fluid in the cooling loop to the fuel-tankheat exchanger to transfer heat between the fluid in the cooling loopand fuel stored in the first fuel tank.
 19. The method of claim 18,further comprising selectively varying a flow of the fluid in thecooling loop through the air-stream heat exchanger and a flow of thefluid through the fuel-tank heat exchanger to maintain a temperature ofthe first heat source below a predetermined heat load temperature. 20.The method of claim 19, wherein the thermal-storage fuel system furtherincludes a second fuel tank in fluid communication with the first fueltank and an engine-fuel unit configured to receive fuel from the secondfuel tank and deliver the fuel to an engine and the engine-fuel unit isin thermal communication with the cooling loop, and the method furthercomprises conducting the fluid in the cooling loop to the engine-fuelunit before conducting the fluid to the air-stream heat exchanger totransfer heat between the fluid in the cooling loop and the fuel in theengine-fuel unit.